Merge pull request #12 from dmeyer3691/master

Addresses issue #9, Refactoring of test_freshdata.py
Idiomatic messages for exceptions
Added logic to require keys. Default key to use
for calls to 'us census' in test suite. Changed
`source` class API to use dicts as defaults
for parameters. Added parameters for fixed defaults.
changed manager __init__ API to use source_name rather than source
Added global flag to test suite to limit network calls
Adresses issue #11 and refactors code for compliance with pep8