Allow specifying a class blacklist, and only enable for non-blacklisted objects

* The cost of favoring double-click is a short delay when handling click. For some objects on the map that's annoying and uneccesary, like zoom buttons and popup close buttons. Allow specifying a "blacklist" of Leaflet classes for which "favor double-click" won't apply.
* The default blacklist includes `L.Popup` and `L.Control` (which represents e.g. buttons on the map).
* Add functions to get and set the blacklist and the delay, rather than specifying them on `enable`.
* Update demo to discuss this delay.
Merge pull request #2 from azavea/allow-class-blacklist

Allow specifying a class blacklist, and only enable for non-blacklisted objects