[xBuild] Support ToolsVersion 12

From VS2013, MSBuild is now decouple from the .NET Framework and is a separate redistributable. This means it has a new location and version numbering scheme. There are also some addition MSBuild properties because of this
[xBuild] Support use of properties defined in Choose elements in project references

This is used by the .fsproj files generated by VS2013 which use a Choose element to set the location of the F# targets file to import
[xBuild] Suport >= in string conditions

e.g: '$(MSBuildAssemblyVersion)' == '' and ('$(VisualStudioVersion)' != '' and '$(VisualStudioVersion)' >= '12.0'), used in the .targets files shipped with VS2013
[xBuild] Support for ImportGroup element

Used in the .targets files shipped with VS2013
[xBuild] Fix Import elements with the combination of wildcards and absolute paths

Used in the .targets files shipped with VS2013