Generated code changes for previous commit (basically InternalBuildGeneratedFileFrom => FromGeneratedCode)

Other changes are due to the well-known types changing without us regenerating.

This requires .NET 4.5, and there are a few compatibility changes required around reflection.
Creating a PR from this to see how our CI systems handle it. Will want to add more documentation,
validation and probably tests before merging.

This is in aid of issue #590.

We'll see what I've missed when CI fails...

Generated code changes from previous commit

Note that now we need a proto3 version of addressbook.proto. This may affect other platforms, and could do with an overhaul to follow proto3 conventions anyway (e.g. repeated field names). Will need to think about that carefully before merging into master. Raised issue #565 for this.

Everything should now be consistent - the only difference between files is the year (aside from generated files).

Fixes issue #531.

Prod code works, but some tests are broken. Obviously those need fixing, then more tests,
and review benchmarks.

- Remove some old proto2-based C#-only messages
- Remove the "build" directory which only contained out-of-date files
- Remove the csharp_namespace option from proto2 messages
- Change "Google.ProtocolBuffers" to "Google.Protobuf" in other messages

(Primarily this is starting the hash code of messages at a non-zero value...)

Fixes issue #523.

Fixes issue #527.

- Make some members internal
- Remove a lot of FrameworkPortability that isn't required
- Start adding documentation comments
- Remove some more group-based members
- Not passing in "the last tag read" into Read*Array, g