tests with mock + py3 fixes + api_key for download

[scisco]

I've made a few changes so I can use the library in two of my projects:

The changes are:

• used a simpler function for xml tree compare tests instead of using Django
• done some linting and Python 3 fixes
• added apy_key as an argument to two of the functions
• added mocks for a few tests
• save api_key to ~/.usgs instead of /tmp/usgs

Feel free to merge if they are helpful. Thanks.

Also, the few tests that are not skipped pass on travis if you turn it on. Here are the results: https://travis-ci.org/developmentseed/usgs

[kapadia]

Hi @scisco - thanks for these updates. Do you mind changing all instances of usgs2 back to usgs? I'll then merge, and push to PyPI.

[scisco]

@kapadia done

[kapadia]

Thanks @scisco. Much appreciated, especially removing the Django xml comparison.