Create log handlers from clients - #103

This changeset allows users to create log handlers using clients, and customize reports before they are sent to Bugsnag.

To create a client and handler:

client = Client(api_key='...')
handler = client.log_handler()

logger.addHandler(handler)

To alter add or remove information from log records before a report is sent to Bugsnag, use callbacks:

def add_custom_fields(record, options):
    if 'meta_data' not in options:
        options['meta_data'] = {}

    options['meta_data']['fruit'] ={'grape_count': len(record.grapes)}

handler.add_callback(add_custom_fields)

All records emitted by the logger are then sent to Bugsnag using the client.

To filter the logs sent, use logging.Filter:

class MyFilter(logging.Filter):
    def filter(self, record):
        return record.account_id != 0

handler.addFilter(MyFilter())

The existing options to pass an api_key and extra_fields to BugsnagHandler have been deprecated in favor of client-based handling