Js vcategories to tags

@georgehrke this one's for you ;) Well at least the edit dialog is.

This is a JS version of the OCP\ITags interface and an edit dialog to add/remove tags.
It replaces the oc-vcategories.js thingy that didn't work really well.
Currently only used by calendar and contacts.
Refs: owncloud/calendar#112 (PR will follow shortly for that one)

@bartv2