Implemented a more robust filtering mechanism

[coryvirok]

You can now add custom filtering logic to determine if the error/message/payload
should be sent to Rollbar.

e.g. Only send messages that match a regular expression:

import re

import rollbar
from rollbar import events

def ignore_unimportant_messages(message, **kw):
    if re.search(r'everthing is broken', message):
        return message
    return False

events.add_message_handler(ignore_unimportant_messages)

# This will not report to Rollbar
rollbar.report_message('everything is fine...')
# This will report to Rollbar
rollbar.report_message('everything is broken')

@brianr cc @Crisfole

[rivkahstandig3636]

@rokob Could you please review this?

[rokob]

I rebased this on master and fixed it up to pass tests. It looks good to me for adding this functionality. This entire library needs a lot of refactoring so adding this in this way seems reasonable at this time to get this PR out of the queue.