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- Remove the "build" directory which only contained out-of-date files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

Convert both the full and lite runtimes (and json/xml serialization assemblies) to be Portable Class Libraries.
All referring projects are now .NET 4 client rather than .NET 3.5.
This commit also fixes up the ProtoBench app, which I'd neglected in previous commits. (Disentangling the two sets of changes would be time-consuming.)

Move to a single solution file containing all of the C# projects, but no other solution folders - it's easier to edit those files outside VS than keep adding and removing them from the project.
The AddressBook protos have been regenerated (with a change to the example proto which I haven't included in this change - I'll wait for us to decide exactly what we're doing with namespaces before changing protos outside the csharp directory.

Note that now we've got Addressbook.cs which contains AddressBook and Addressbook classes. It's bad enough that we've got a class called AddressBook within a namespace of AddressBook (hard to get away from) but having things vary just by case is nasty.
This is more evidence that an option for renaming the file and descriptor class would be welcome. (A single option can probably handle both.)