#506 - rss_parser unit tests

This contains tests for `freeseer/plugins/importer/rss_feedparser/__init__.py`.  It uses a local JSON file as the expected output and is compared to the actual output from `get_presentations()`.

__Tasks:__
- [x] Create local `presentation_feed.rss` file
- [x] Create local `presentation_feed.json` file
- [x] Mock access to feed URL
- [x] Test if `presentation_feed.rss` and `presentation_feed.json` match outputs
- [x] Clean up PR & squash

Project proposal: https://docs.google.com/document/d/1_-vzW36GFCA23Wm415Xyf7B2DNM_-I6DFc6i4s5zGnc/edit#