Add util function to mask credit card from xml so we can log it

Some changes are coming together in this pull request:

- Credit card number masking before logging the request payload:
`<CardNumber>1234567890123456</CardNumber>` will become `<CardNumber>123456******3456</CardNumber>` 

- Removed unused and out of date billet code and XML files
- Improved payment plan's variables / constants
- Renamed ```GenericRequest``` to ```BaseRequest``` and ```make_request``` to ```fetch```
- Added ```asyncreplay.py```, which is being used in some tests to replay their respect requests
- Removed the tests timeout variable from ```base.ASYNC_TIMEOUT``` and now set the environment variable ```ASYNC_TEST_TIMEOUT``` on the ```Makefile``` -- the timeout set by this variable is globally available to every ```AsyncHTTPClient.fetch``` calls
- Make nosetests stop as soon as a test fails