Packages and project file cleanup

This does a lot of cleanup of the packages and projects.

1. Delete the JSON.NET packages folder. Until JSON.NET supports the Monotouch and Monodroid profiles, we need to reference JSON.NET from the ext folder.
2. Remove the Microsoft.Bcl.* build targets and packages. According to @xpaulbettsx we no longer need these since we target .NET 4.5 or later.
3. Fix the WP8 csproj files. These were missing project configuration sections. My changes seems to fix them. At least on my machine.

This is ready to be merged.

There's still some more work to be done, but I'll do it in another pull request.

__Open Questions__

Some, but not all, of the project files have a bunch of Code Contracts config. Should this be here? For example:

```
<CodeContractsEnableRuntimeChecking>True</CodeContractsEnableRuntimeChecking>
    <CodeContractsRuntimeOnlyPublicSurface>False</CodeContractsRuntimeOnlyPublicSurface>
    <CodeContractsRuntimeThrowOnFailure>True</CodeContractsRuntimeThrowOnFailure>
```

If so, should it be in every project file?