Fix #151 Make RSS / CSV Loaders plugins

Refactored plugin.py to add importer plugin category. Refactored
database.py to use rss and csv plugins. Added tests for rss and csv
importers to test_database.py.

Closes #413
Added base flake8 config
Removed old pep8 unittests
Added flake8 to travis config
Merge branch 'pep8_check' of github.com:mtomwing/freeseer into pep8_check