Documentations and reorganizing
Removing unused packages
Fix indexing bug for sensor list
Add ability to specify start and end date for downloading data
Tests for CDEC module.

Not all tests pass when mocking is used because one of the tests,
`test_get_station_data` needs to read two data files and the mocking
causes the wrong data file to be read.

So at the moment this will pass:

    ULMO_DONT_MOCK_TESTS=1 py.test test/cdec_historical_test.py

but this will not:

    py.test test/cdec_historical_test.py