Updated Data Scrubbing

[matthew-gerstman]

⚠️ Need to update documentation to reflect changes.

Issue #9

Updated the data scrubbing code to allow for users to include their own custom processors. It also validates against a few more "bad keys" such as "authorization", "apikey", and "api_key." I took these keys from raven-python

I also added value validation to scrub out anything that looks like a credit card number.
Also it's now scrubbing HTTP Headers.

This is a more thorough implementation of: http://sentry.readthedocs.org/en/latest/developer/client/#scrubbing-data

Note: This is an API breaking change as NewClient now takes a ClientConfig struct to handle both tags/processors

Edit:
I wrote a processor_tests.go file that includes a test of the default processor and a custom provided one.