Allow extra record fields in BugsnagHandler

This PR allows you to tell BugsnagHandler to add stuff to meta_data for you when calling bugsnag.notify().  This is really useful in the event that you're attaching some extra attributes to your LogRecord 's as is [described in the python cookbook](https://docs.python.org/3.4/howto/logging-cookbook.html#using-filters-to-impart-contextual-information).   It prevents you from having to implement your own LoggerHandler or LoggerAdapter when you want to do this.  You simply give your Bugsnag handler an extra_fields dictionary, where the keys are sections you want in meta_data and the values are attributes of your LogRecord.  If the attributes exist on the LogRecord, they are automatically added to the appropriate sections in meta_data.  It also maintains the old incorrectly documented behavior of putting some default LogRecord attributes in a section called "EXTRA DATA".