Implement basic notification/interaction machinery.

Get notifications from Taverna Server and present the linked interactions in an
iframe within the individual run page. Interactions are posted one at a time,
even if they are read from the feed at the same time, to avoid hitting this
bug/behaviour: https://jira.biovel.eu/browse/TAV-311
Proxy secure notifications/interactions through Player.

Interactions are completed by publishing a reply to the notification feed and
sending data to its associated working directory. Both the feed and the working
directory are secured with the standard Taverna Server credentials so we need to
proxy all this communication through the Rails app (Player) so that we can use
the correct credentials.
Merge branch 'rh/int'

* rh/int:
  Proxy secure notifications/interactions through Player.
  Implement basic notification/interaction machinery.
Add state tests to the run model.

Run#running? and Run#finished? are useful shortcuts to have.