* Compiler changes (extensions)

* Generated changes (extensions)

* Library changes (extensions)

* Adjusted a summary to indicate ContainingType can be null for extensions

* Compiler changes (custom option review + access level review)

* Generated code changes (custom options + access review)

* Library changes (custom options + access review)

* Support C# 6 with library changes

* Access HasValue by property

* Set access level of all extension classes to internal (revert in next PR)

* Added null checks to custom options

* Rebase on master and regenerate Conformance

* Removed second dictionary from ExtensionSet

* Rebased compiler changes

* Rebased generated code changes

* Rebased library changes + review changes

* Add more safety checks to extension accessors

* Remove instances where extension sets were unnecessarily allocated

* Remove cleared items from sets
Empty sets are now made null
IExtensionMessage -> IExtendableMessage

* Remove dead code from IExtensionValue impls

* Clean both repeated and single value extensions

* Add GetOrRegister method for repeated fields and allow clearing repeated extensions

* Add type safe ClearExtension methods, remove non-generic IExtendableMessage interface.

* Simplify ExtensionSet.TryMergeFieldFrom

* Rebase on master to resolve conflicts

* Fix Makefile.am

* Add ObjectIntPair to Makefile.am

* Modify how end tags are encounted in merge code (compiler)

* Modify how end tags are encounted in merge code (generated)

* Modify how end tags are encounted in merge code (library)

* Regenerate generated code through generate_descriptor_proto.sh

* Modify how end tags are encounted in merge code (compiler)

* Modify how end tags are encounted in merge code (generated)

* Modify how end tags are encounted in merge code (library)

* Regenerate generated code through generate_descriptor_proto.sh

Add group support for proto2

* Compiler changes

* Generated code changes

* Library changes

* Compiler style changes

* Generated style changes

* Fix Windows build errors

* Implement changes from review

* Reintroduce proto2 check

* Compiler changes (required handling review)

* Generated code changes (required handling review)

* Library changes (required handling review

* Field presence rewrite (compiler changes)

* Field presence rewrite (generated code changes)

* Compiler comment

* IFieldAccessor.HasValue library implementation

* Remove Clear methods and default values from proto3 code (Compiler)

* Remove Clear methods and default values from proto3 code (Generated)

* Remove Clear methods and default values from proto3 code (Library)

* Fix distcheck error

* Rewrite default string values to use base64 and convert

* Library changes (IMessage2)

* Compiler changes (IMessage2)

* Generated changes (IMessage2)

* Rebased and regenerated

* Compiler changes (initialized extension)

* Generated changes (initialized extension)

* Library changes (initialized extension)

* Refactor MessageExtensions.IsRequired

* Move string default value creator and bytes default value creator back to seperate methods

* Dead code cleanup

* Fixed segmentation fault
Removed unused header method declarations

* Edited MessageFieldGenerator to clone to backing field instead of property

* Generated C# proto code

Add preserve UnknownFields
Compare floating point values bitwise
Add auto-generated header to C# generated files

* Add auto-generated header to C# generated files

* regenerate C# protos

 Add unknown field support for csharp

This consists of:
- Changing the codegen for the fixed set of options protos, to parse unknown fields instead of skipping them
- Add a new CustomOptions type in the C# support library
- Expose CustomOptions properties from the immutable proto wrappers in the support library

Only single-value options are currently supported, and fetching options values requires getting the type right
and knowing the field number. Both of these can be addressed at a later time.

Fixes #2143, at least as a first pass.