PCL support (see #15).

Consequently, ParseException no longer supports serialization, regular expressions are no longer compiled, and an extension method has been implemented to support treating a string as IEnumerable<char> (which is not possible in WinRT's portable subset).

Also updated NuGet.exe and the build script to produce a working PCL NuGet package.
Merge pull request #38 from kentcb/master

PCL support (see #15).