Improve the Processor base class and SanitizePasswordsProcessor class

Improve Processor to also check for stacktrace at the 'data' level.  A stacktrace can exist in the data
even without an 'exception' key.
Also added 'pw' and 'cred' to the list of FIELDS to screen for in SanitizePasswordProcessor.  These
two fields being used in the standard python LDAP module.
Are you asking for updating the tests/processors/tests.py for case when an event that has 'stacktrace' in its data?  I have just updated the processor test to include the two additional 'pw' and 'cred' field.  As for the test case for 'AUTO_LOG_STACKS', I will need to spend a bit more time in figuring how to write a test for that (turn on auto_log_stacks and build a raven.events.Message).

This can be reproduce when "auto_log_stacks" option is set to True and logging a Message event.