Added BouncyCastle.sln and csproj which are in VS2010 format

Target .NET 2.0 instead of 1.1 since no one uses 1.1 anymore.

Also don't compile the unit tests & data into the crypto assembly, instead create a separate UnitTests project/assembly for those.

This makes it easier to integrate BouncyCastle into other projects and builds nicely with both Xamarin Studio (aka MonoDevelop) and Visual Studio 2010+.