Change file(), files() to get_file, iter_files()

Reflects changes implemented in 1.0 refactor

https://github.com/jjjake/ia-wrapper/commit/05f4378c977247c264ffab6fe213511717d83053#diff-538a4614642d430cc478b9e1862232a0
Merge pull request #74 from wcaleb/doc-fixes

Update Python module documentation for downloading