new BaseSection model to make subclassing easier and more flexible
fix comment typo
increase Section slug length to 200. Resolves #43

`slug` is 200 and not 255 because `full_slug` is already capped at 255 and this leaves room for a level or two of hierarchy.
revert migration. we'll leave the migration until the release prep and bundle any changes into one single migration.
Section model gets a unique `full_slug`. Resolves #84
more descriptive comment tags
test on new Django releases
Merge branch 'basesection_model'
Merge branch 'unique_full_slug'
Merge branch 'increase_slug_length'

Conflicts:
	armstrong/core/arm_sections/models.py
	tests/models.py
Add Coveralls. Add badges for TravisCI build status, Coveralls, PyPi version and license.