Adds the NFluent.PCL project (with profile 136) and introduces #ifdef (MOBILE) wherever we need it to make it compile.

So far, and as a consequence, the PCL version won't have Check.ThatCode() statements and no dedicated unit tests (to be improved thus).
Updates the build script and the nuspec file in order to distribute the PCL version.
Includes XML documentation generation within the NFluent.PCL project.
Fixes some warning within the PCL project and treat warning as error for this project.
Updates the release note which now indicates that NFluent is now available as a PCL: portable-net4+sl5+wp8+win8+MonoMac+MonoTouch+MonoAndroid
Updates the release note to fix typo.