Normalize add/remove to support multiple args

For browsers that natively support classList, normalizes add/remove to
always support adding or removing multiple classes at a time. Affects
Firefox < 26 and IE 10/11.
Normalize toggle API to take second argument

For browsers that natively support classList, normalizes toggle
implementations so they all support the second argument. Affects
Firefox < 24 and IE 10.
Merge pull request #28 from eanakashima/normalize

Normalize add, remove, and toggle to support multiple arguments