Add support for severity

[the21st]

ILoggingEvent.getLevel() is not reflected in the event sent to bugsnag. INFO level is reported as ERROR to bugsnag.

[SierraGolf]

Hey, since I initially created this repository Bugsnag has updated its API. In order to support the severity an update to version 2 of the notifier API would be required. As far as I can see this is mostly about re-arranging fields and about adding support for some new fields like the severity.

I think a minimal effort to re-arrange existing properties and leaving out any optional properties would be fine for the first iteration.

About supporting severity, Bugsnag only supports info, warn and error. How would you handle/map the remaining levels, which are OFF, DEBUG, TRACE, ALL?

Pull requests are welcome!