Added isPhone property from WinJS 2.1

Added WinJS 2.1 isPhone property to support WinJS 2.0 and WinJS 2.1 code
sharing in "Universal" Shared projects.

See http://msdn.microsoft.com/en-us/library/windows/apps/dn607959.aspx.
Merge pull request #2399 from craigktreasure/master

Added isPhone property from WinJS 2.1