Added .NET 2.0 version (with LinqBridge)

I use ElfSharp in [Zero Install](https://github.com/0install/0install-win). To ensure it runs out-of-the-box without additional dependency installations on most Windows boxes it still targets .NET 2.0.

I added an additional project file to ElfSharp that targets .NET 2.0. In order to replace the missing LINQ functionality I used [LinqBridge](https://code.google.com/p/linqbridge/). The other compatibility changes were rather minor.