Fix to allow opening C# and F# projects newly created in VS2013

This is not the same as https://github.com/mono/monodevelop/pull/407
F# projects newly created in VS2013 have one extra difference from VS2012 projects upgraded to VS2013: the `ToolsVersion` on the `.fsproj` is set to `12.0` instead of `4.0`. C# projects newly created in VS2013 also have this problem (although there were no extra problem that affected both upgraded and new solutions like in F#)

Currently MonoDevelop has the tools version strongly connected to the .Net version, and VS2013/MSBuild12 breaks that. I tried to create a `MSBuildFileFormatVS13` with the right tools version, but then there were multiple problems because MonoDevelop was trying to find .NET 12.0, so I just did the same that was already done for `SlnVersion` and declared `MSBuildFileFormatVS12` to support both tools version `4.0` and `12.0`. It's a bit of a hack, but it works. I also renamed it to `MSBuildFileFormatVS13And13` to be more clear what it does. I did a similar fix for xBuild (https://github.com/mono/mono/pull/767) and there I was able to do it correctly, and with some work it's also possible to do the same for MonoDevelop, but as VS2013 RTM is already out and a lot of people are potentially hitting this problem, it would be nice to be able to get this temporary fix released even if there are plans to fix it in a better way later on.