Added logging support to Hangfire

Covers issues raised in issue #218, by augmenting the cancellation token with a more feature-ful JobExecutionContext (which inherits the cancellationtoken for backwards compatibility reasons).  

JobExecutionContext offers an API for logging and accessing the Job Id.

The dashboard displays logged messages as seen here:

![capture](https://cloud.githubusercontent.com/assets/435529/4760281/a1aa4da0-5aee-11e4-8ec4-1edc33b63136.PNG)