Add XML and Async support to Portable and WP

NOTE: This commit requires the .NET 4.0.3 Multi-Targeting Pack, found
here (http://www.microsoft.com/en-us/download/details.aspx?id=29052)
- Removed support for .NET 4.0 in the Portable project, because .NET
4.0.3 includes System.Xml.Linq.
- Removed support for Windows Phone 7.0 from the Portable project,
because 7.0 apps can no longer be submitted to the Windows Phone Store.
- Added Microsoft.Bcl & Microsoft.Bcl.Async to Portable and Windows
Phone, to bring type parity and async support across all platforms.
- Adjusted the compilation constants to include XNode and Async support
in Portable and Windows Phone, for converting Xml to JSON.
- Adjusted the compilation constants to include the proper Unit Tests
for Portable and Windows Phone.
- All unit tests pass (NOTE: Microsoft.Bcl.Build adds some additional
info, meaning the tests pass in the VS2012 IDE but not in the build
scripts, so you'll have to make some changes.)
- Updated the build script to change the NuGet package to match the
Portable and Windows Phone profile changes, and increment the version
number.