Move to dotnet cli for building, and .NET Core (netstandard1.0) as target platform

This also updates the version number to 3.0.0-beta4

* Improve exception throwing implementation in collections

* Implement RepeatedField.AddRange.

This fixes issue #1730.

* Optimize AddRange for sequences implementing ICollection

(Also fix a few more C# 6-isms.)

* Remove the overload for Add(RepeatedField<T>)

We now just perform the optimization within AddRange itself.

This is a breaking change in terms of "drop in the DLL", but is
source compatible, which should be fine.

Fix a <package>_FIND_VERSION_PRERELEASE being ignored

Remove the baseline files from the make dist file list.

https://github.com/google/protobuf/pull/1762 remove the files, but I forgot to
update Makefile.am to remove them.

Modify csharp README since there are now two NuGet packages

Remove unnecessary reflection call

This is the only call to TypeExtensions.IsValueType, so we can remove
that method, making the whole type conditionally compiled out for .NET 3.5

Drop the performace baselines.

We weren't really using them, and the nested path causes checkout problems
on windows.

Don't #import the .m files.

Change placement of DebuggerNonUserCodeAttribute

Make sure also Solaris x86 gets PATH_MAX

This does not affect the generated code.
If we decide we want to apply attributes to generated types, we should start by
just reverting this change.

Fix problems detected when integrating the code to our internal repo.

As bazel folks are looking at getting auto generation of module maps going and
the importing of sources files causes issues there.  We were only do it to
hack around some of the apple linker behaviors around objc classes and
categories, but even that isn't complete and CocoaPods was already doing -ObjC,
and developers not using pods could have still needed it to ensure everything
was linked anyways; so drop the hack of importing sources.

Fix spelling

Fixed failing JS tests

A couple of small tweaks were needed to fix test failures that were
inadvertently introduced in the last down integration. I also had to add
the map.js file which was missing.

add nullable qualifier to return types that can be nil, to support Swift 2 try-catch syntax

Add the `nullable` qualifier to return types of Objective-C methods that
can return a nil due to errors. This change makes these methods
compatible with the Swift 2 try-catch syntax.

I think this has caught everything.
I've left a stub for attributes to be applied to the types themselves, but we don't currently need anything.

Follow-up commit will include the changes to generated code itself.

Fixes #1671.

Add JsonParseOptions to ignore unknown fields

Fix some failing travis tests.

1. Add missing header file to Makefile.am.
2. Re-generate objectivec generated code for well-known types.

Change-Id: If28217c701cf8bd739ea0db240e9eee600f23ee7

Add minified Json printing functionality

Xcode project cleanup/setup.

Change-Id: Ibf7d1df850f4b497303f9f617751be98327898ef

In Python 2.6 the positional argument specifiers are apparently required
to be explicitly specified:
http://stackoverflow.com/questions/10054122/valueerror-zero-length-field-name-in-format-python

This includes all internal changes from around May 20 to now.