Mono build fixes

This is related to https://github.com/bamboo/boo/pull/75

I'm investigating all this, here are my findings so far:

  - MonoDevelop creates a `.NETFramework,Version=v4.0.AssemblyAttribute.cs` file in the `obj` directory of each folder when building the solution
  - `nant` launches `scripts/update-vs2005-env.boo`, which updates the .csproj files to include any .cs file in the src. This means that if those `.NetFramework` files are there it will include them in the projects.
  - `.booproj` files *must* include the attribute `ToolsVersion="4.0"` in the root `<Project>` tag
  - xbuild doesn't seem to support the syntax `$([Namespace::Class::StaticMethod()])`
 
 In my tests with mono and .net the results are:

  - Both create a file named `obj\Debug\.NETFramework,Version=v4.0.AssemblyAttribute.boo`
  - msbuild includes this file in the compilation, xbuild does not