Also support Visual Studio 2013.

This commit adds support for Visual Studio 2013 to SaveAllTheTime.
According to [this post][1], the version number format has changed for VS
2013, so that a single version number now means "support this specific
version only", much like [##.#] did before. The same behaviour as before
can be achieved by specifying a supported version of [##.#,].

I have chosen to explicitly specify VS2012 and VS2013 only, as I have no
idea whether or not SaveAllTheTime will continue to work with the next
version of Visual Studio after VS2013.

  [1]: http://blogs.msdn.com/b/visualstudio/archive/2013/08/08/update-for-extension-authors-vsix-manifest-version-ranges.aspx,
Merge pull request #26 from alastairs/vs2013-support

Also support Visual Studio 2013.