Find msbuild.exe in new location #126

Pullrequest fixes  #126. Trying to read the msbuild.exe locations from registry instead of folder based look-up to have a consistent look-up strategy.
Why not use the ToolsVersions\12.0\MSBuildToolsPath key, instead of the 12.0\MSBuildOverrideTasksPath key? The MSBuildToolsPath keys are mentioned as being supported by MSFT on http://blogs.msdn.com/b/visualstudio/archive/2013/07/24/msbuild-is-now-part-of-visual-studio.aspx

On my machine (vs2013 + vs2010sp1), 3.5\MSBuildOverrideTasksPath is undefined.