Added invitation model
Added an admin for the invitation model
New mixins to require that a competition be open or running (or both)
Started working on invitation views and tests
Merge branch 'develop' into feature/invitations

Conflicts:
	src/competition/views/mixins.py
Added a spot to base.html to show who we're logged in as
Got working invitations and cleaned up a bit
Generalized CheckAllowedMixin a little

Now there's a "was_not_allowed()" method so that we can specify what
to do when the user isn't allowed to do something
Added filter_horizontal to members in team admin
Aded get_latest_by attributes to Team and Competition
Now displaying non_field_errors in horizontal_form.inc
Added a method to Team to calculate the number of invites a Team has left
Fixed an interitance dumb in a couple of view mixins
Added a message to invitation_create.html

When a user has no teams to invite people to, (they're all closed,
they're all full, whatever), display a "you can't invite anyone right
now" message on the page
Added accept/decline functionality

Need to finish writing tests
Added django-admin-tools to helper project

It's pretty snazzy.
Added Invitation to models module
Created InvvitationFactory
InvitationDetailView now marks invitations as read
Worked on a couple of invitation tests