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.
Couldn't handle messages with no "from" attribute.
Some refactoring to add code that checks if a messages is a headline to the
utils module.
Updated tests. Add sinon so that we can test returned value of spy.
