.NET 3.5 support

This branch adds .NET 3.5 support for the REST client features to this project. The REST server features are omitted from the .NET 3.5 build.

To build the NuGet package, first build the solution with a release configuration and then use a command like the following:

```
NuGet.exe pack SimpleRestServices.nuspec -Version {version} -Symbols -Properties Configuration=Release
```