Publish Extempore sequence / progression data to queue

[oubiwann]

Currently, some custom Extempore code is sending this data structure to LFE:

                #m(tempo ,tempo
                   chord-pitch ,chord-pitch
                   chord-type ,chord-type
                   beat ,beat
                   beats-per-note ,beats-per-note
                   note ,note
                   note-duration ,dur))

This needs to be published to the inbox of a gen_server; tasks include:

• [ ] Setting up https://github.com/cabol/erlbus
• [ ] Telling the gen_server to subscribe to a new topic (new API function)
• [ ] Updating the handle_info callback (which receives this data from Extempore) in undertone.extempore to publish to a given topic (auto-generated via a unique id in Extempore)
• [ ] Creating a new function in undertone.web.api for getting a list of all topics to which the undertone.extempore gen_server is subscribed
• [ ] Create a new page that lists all the topics, hyper-linked to their own pages
• [ ] Updating the phrase-data function in undertone.web.api to subscribe to a particular topic
• [ ] Having phrase-data's long-polling mechanism pull all messages off the queue, and drop all but latest (the latest will get sent to the browser)