Added IsRootPath function
Added IsRootPath function

Can be improved by further investigating edge cases that Path.GetDirectoryName returns null in.

rainersigwald/msbuild,jeffkl/msbuild,jeffkl/msbuild,mono/msbuild,rainersigwald/msbuild,cdmihai/msbuild,sean-gilliam/msbuild,chipitsine/msbuild,cdmihai/msbuild,AndyGerlicher/msbuild,cdmihai/msbuild,mono/msbuild,chipitsine/msbuild,rainersigwald/msbuild,AndyGerlicher/msbuild,jeffkl/msbuild,AndyGerlicher/msbuild,sean-gilliam/msbuild,cdmihai/msbuild,mono/msbuild,chipitsine/msbuild,mono/msbuild,AndyGerlicher/msbuild,sean-gilliam/msbuild,jeffkl/msbuild,sean-gilliam/msbuild,chipitsine/msbuild,rainersigwald/msbuild