lib/bugsnag/notification: delete the unused ex_class lvar
lib/bugsnag/middleware_stack: do not shadow the variable
lib/bugsnag/middleware_stack: [] -> call

The code is already complex enough and
[] can be confused with Array#[].
lib/bugsnag/notification: add the ability to ignore notifications
lib/bugsnag/notification: return after the callbacks

Fixes #147 (Before bugsnag notify should allow you not to send)
Merge branch '149-before-notify-ability-to-ignore'
Merge branch 'delete-unused-variable' of github.com:bugsnag/bugsnag-ruby