Made the Tracker methods (optionally) thread-safe for MVC FilterAttribute-based Tracker instances.

The use of class-level properties without thread locks means that
multiple simultaneous requests have the possibility of overwriting
eachother's CharacterSet, Hostname, and Language parameters. Moving them
to optional parameters allows the HttpContext extension methods to
correctly pass the correct values to a static Tracker.
Added support for ASP.NET WebAPI.