Add an `addAndRemove` method on set.

Here's a way to make batch insert/removes from a set and
fire events only once everything has happened.
Allow and prevent can take multiple arguments
Fire with the same argument signature as add/remove
Make sure events aren't prevented
Don't test for 'change' since SetProxy doesn't forward it.
Documentation for addAndRemove.
addAndRemove implementation without prevent/allow

Wrapped new _add and _remove methods
Revert prevent/allow

:'(
Merge pull request #738 from Shopify/set-add-and-remove

Add an `addAndRemove` method on set.