Added infer.NET package nuget reference. Added some code doc. Other minor changes

Added infer.NET package nuget reference. The actual binaries aren't pushed due to licensing issues, but the IDE should automatically download them (Visual Studio 2013 definitely does, not tested in Xamarin).

Added some code doc. 

Minor changes - added a couple of extra Infer.NET variable names to aid factor graph, Moved engine definition to after model definition.