Fixed msbuild compilation by removing the 3.5 bootstrap ItemGroups. (VS2010 added those on its own - hey, we are targeting v2.0 and we are not using ClickOnce!)
* Source/Build.Tasks/Build.Tasks.csproj:
* Source/Examples/OpenTK.Examples.csproj:
* Source/GLControl/OpenTK.GLControl.csproj:
* Source/Compatibility/OpenTK.Compatibility.csproj: Ensure we are
using the 2.0 toolset to build. Fixes compilation errors on
MonoDevelop/xbuild.
* Documentation/Build.Docs.csproj: Use "." instead of
"$(MSBuildProjectDirectory)", because MonoDevelop/xbuild fails to
replace the latter with its value.