Add support for 'block syntax' on Bugsnag.notify calls

[loopj]

This allows users to customize the error report sent to Bugsnag using ruby block syntax on each Bugsnag.notify call as follows:

Bugsnag.notify(ex) do |notification|
  notification.severity = "info"
end

As well as being an arguably cleaner syntax than before, this also exposes the full power of accessing the Notification object to every Bugsnag.notify call - provideing a single interface to error report customization. This means Bugsnag.notify report customization and before_notify_callbacks can all share a single documentation.

This PR is the super un-opinionated version, which does not break existing APIs.

I actually propose we remove all the slightly hacky overrides logic and replace with this, which would be an api-incompatible change requiring a major version bump - any thoughts?

[snmaynard]

This looks like a good approach to me. I've always felt that notify_or_ignore should be what notify does. If we merge this and bump the major version, how do you feel about making notify and notify_or_ignore do the same thing?

[kattrali]

Looks great to me, and seems more Rubyish. Thanks, @loopj!