Make @ratelimit stackable.

Example included!
Merge pull request #17 from rlr/stackable

Make @ratelimit stackable.
Add `is_ratelimited` helper function
fix formatting
pep8 fixes
More formatting stuff
Merge pull request #20 from rehandalal/helper-function

Add `is_ratelimited` helper function
created RateLimitMixin (incl. Tests)
fix - Dict Comprehensions not available in python < 2.7
fix - unittest.SkipIf not available in python < 2.7
Merge pull request #21 from mfelsche/feature/cbv_mixin

created RateLimitMixin (incl. Tests)
Fix tox/travis so that they test the lastest release of each minor version.
Add a note to contributing doc to install tox first and make example commands more consistant.
Add note about backwards compatibility import.
Merge with master.