Added Event.collaborators M2M with Participant, plus migration, updates
to corresponding object accessors, and screen changes to show approved
collaborators as well as owner participant against each event.
Merge branch 'master' into event_collaborators

Conflicts:
	eluminate_web/templates/events/_event_snippet.html
Applied |striptags|urlize|linebreaks|truncatewords filter to
event.description on participant detail page.
In Events lists, as well as Events whose owner participant is in the
selected category, show alse all Events where one or more of their
approved collaborator Participants are in the selected category.
Prevent a collaborator being set as Event owner, and vice-versa.
Merge pull request #152 from jogwen/event_collaborators

Event collaborators model change #127
Each Event can now have a set of collaborating Participants as well as the owner Participant. 
A south migration is included to fix any existing database (ie to add the table, so won't create any relationships). 
All screens listing Events now display Events collaborated upon by relevant, approved Participants as well as those Events owned by relevant, approved Participants. New Participant model method approved_collaboration_events helps with this.
All screens showing Event summary/details now display approved collaborating Participants as well as the owner Participant - new Event model method approved_collaborators helps with this. Also made more things clickable in Event summaries (to click through to Event detail).
Event.save() and a signal handler on Event.collaborators M2M field prevent an Event owner becoming a collaborator, and vice-versa.