Allow vsix2resharper to be opened in VS 2012

This changes the vsix2resharper.csproj file such that it can be opened
by both Visual Studio 2012 and 2013.  The source.extension.manifest file
is compatible with both versions so the produced VSIX will be valid.
Merge pull request #1 from jaredpar/build-vs2012

Allow vsix2resharper to be opened in VS 2012