This change refactors out the plugin code from converse-core into
src/converse-puggable.js
Additionally, plugins now have an optional attribute `optional_dependencies`
which is an array of dependencies which are "nice-to-have" but not essential.
Work has also been done to ensure that a plugins' dependencies are first loaded
before the plugin itself.
Compare bare JID of from attr in roster update. @teseo's fix was too specific
and tailored for a misbehaving server (servers shouldn't be sending from the
full JID in a roster push).
* Fixed a bug in the process which prevented "to" contacts from being shown as
existing.
* Add "to" or "both" as classes on the contacts to indicate their
subscription status.
* Delete roster handlers in tearDown method to avoid them being registered
multiple times.
