Use setuptools to support develop

Use setuptools so we can do "setup.py develop" to do a develop
install of the package.
Fix broken test and pass creds through env

Removes the check for preferred_version, which isn't present in
Sauce anymore.

Allow user to specify SAUCE_USERNAME, SAUCE_ACCESS_KEY, TEST_JOB_ID
by environment variable.
Add get_job_assets method
Merge branch 'fix-broken-tests'
Merge branch 'assets'