Rearranged project build, single sln/csproj

- We now have one sln and one csproj, with build configurations for
  .NET 2.0, 3.5, 4.0, 4.5
- An additional build configuration produces .NET 4.5 with Entity
  Framework 6. EF6 was brought in via nuget as a dependency, but
  its DLLs are referenced conditionally based on the build config.
- .NET 2.0 uses LINQBridge for extension method and LINQ support
- Removed Visual Studio 2008 support
- Deleted monodevelop mdp/mds, mono xbuild compiles the sln/csproj
  files just fine
- Mono does not build config 3.5, 4.0, 4.5 because they rely on the
  old (non-opensource) Entity Framework. 4.5-EF6 builds fine.
- Mono does not build config 2.0 because of a LINQ/LINQBridge issue
Added Mono.Security for .NET 4.0

The build configurations for .NET 4.0 and 4.5 now reference
the 4.0 version of Mono.Security, while the configurations
for .NET 2.0 and 3.5 reference the 2.0 version as before.
Created .sln per VS version

Important in order to exclude .NET 4.5 from VS2010 and allow
for automatic opening of the correct VS version from explorer
Corrected debug build config of NpgsqlTests
Merge pull request #62 from roji/build

Work on the build system

From @roji "As discussed on the mailing list, rearranged all the build files (sln/csproj) by framework and cleaned up lots of things - look at the commits for details."