Support for XML and Async on Portable and Windows Phone.

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 in the Portable profile.
- 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 to the project, meaning the tests pass in the VS2012 IDE but not in the build
scripts, so you'll have to make some changes to NUnit.)
- Updated the build script to change the NuGet package to match the
Portable and Windows Phone profile changes, and increment the version
number.