[msbuild] improvements

Sorry to bother you again with the msbuild support, but I think I got it right this time :8ball:

  - Will try to load the task dll from BooToolPath, BooBinPath, ToolsPath or the same directory as the .targets file
  - Will look for the compiler (booc, booc.bat, booc.exe) in:
    - BooToolPath
    - Using .Net ToolLocationHelper (not available in Mono)
    - Using the registry for app paths
    - Fall back to just the tool name for the platform

I'm still not sure about how it's managing the resources, I assume that by importing the csharp target all the work is done by it and that it's compatible with Boo. However I don't fully understand how they work on .Net and for my use case (simple embedded resources) works just fine. 

By the way I fixed the compilation in Nant and set the fail flag on it. I usually build my projects with xbuild so it's nice to know when the compilation has failed.

> Tested it with Xamarin Studio, xbuild 3.2 on Mac and msbuild 4 in Windows.