Fixed issues in SLN, FSPROJ and PROPS files

* Merge recent updates from "Dynamo.sln" into "Dynamo.2012.sln"
* Fixed "FScheme.2012.fsproj" to use the right environment variable "ProjectDir" instead of "SolutionDir"
* Made "Dynamo.CS.props" falls back on to Vasari path if Revit 2014 does not exist

NOTE: The change in PROPS file will not affect existing build set-ups