#709 - Animations Addon - Staggering Animation

Initial implementation of staggering animation, single direction or bi-directional as a setting. This is my first time contributing to any open source library really, so I'm putting this up to get feedback and make sure I'm heading in the right direction with this.

Docs - Documentation added for the feature.
Tests - some additional tests for the additions to the ReactTransitionKeySet - but I would like to put some more into ReactTransitionableChild at least. All passing currently.
Code linted.

Definitely have a bit more time to throw at this so let me know if improvements/changes are needed. I feel like I may not have used the KeySets correctly in using an array to get an order (see getOrderedKeys), but it seemed to fall down if I just took a KeySet order on it's word. Correct me if I've made any incorrect assumptions here.

Oh: also, once you're happy with it I'll squash the history on my end so it's a single commit before merging, unless there's a way you folks prefer it done.