First crack at providing instance-based snagging
First crack at providing instance-based snagging

* Add new interface `IBugsnagger`, for instance-based snagging.
* Refactor statically-oriented `Bugsnag` class to use an instance of
  `IBugsnagger` to delegate notification to. This has been done to
  ensure backwards compatibility.
  * `Bugsnag` is still responsible for creating events, but that will
    probably also change in the next steps.

awseward/Bugsnag.NET,awseward/Bugsnag.NET