fix add/remove tags in pile_action_{,un}select

Jquery try to be smart with data coming from html5 data-* attributes [http://api.jquery.com/data/#data-html5] and return a numeric when mid looks like an int. This cause findWhere to not find metadata, ignoring add tags on select and crashing with metadata undefined in unselect.

I guess issues #901 and #886 are related to this.

I'm not sure about the other uses of .data('mid') (or any other "could be seen as numeric" ids)