Major serializer changes

* Since Newtonsoft is having PCL issues, I've implemented a new
interface ICustomSerializer.
* Implemented concrete Newtonsoft and ServiceStackJsonSerializer classes
for the ICustomSerializer interface. ServiceStack does not seem to
suffer from the same issues (yet*) that the NS one does.
* Deleted old unused projects that were not part of the solution.
* Added/tweaked tests.
* Updated buildGib files for new and improved buildGib.