Create test data directory, move tests to tests/flowalert

Closes #18 (closed)

• tests/data/ directory has testing data files using layout described in #18 (closed)
• tests/testdata.py module includes commonly used testing paths and is imported by tests
• tests consolidated in tests/flowalert
• vcr cassettes not modified yet, #23 (closed) may be a better issue for those changes as the code is further refactored

/cc @hschovanec