Add cmd module
Setup interface for CMD wrapper
Fill out form
Refactor filling out form
PadovaCache is an interface to cached results
Generate hashes and check the cache
Fill out CMD input form properly, get HTML response
Parse the CMD HTML reponse to get data URL

The CmdResponseParser class uses HTMLParser to get the URL for the .dat
or .dat.gz file.

The padova/tests/test_parse_result_html.py test verifies this behaviour
using the pre-downloaded HTML response file:
padova/data/cmd_isoc_output.html.
CMD download works!

Runs scripts/test_request.py to show that everything works.

- Updated the cache path generation/lookup to include the native
  extension of the data file (.dat or .dat.gz). This will be useful.
- Use the urllib.urlretrieve function to quickly download a file to
  disk. The file is downloaded into the cache.
- CMD.get() return the cached path once it is downloaded.
- I've verified that caching works; a second call to test_request.py
  will return the cached file path.