Adds invitation by email to events

`initializers/mweb_events.rb` now loads controlers and models from `lib/mweb_events/`

`lib/mconf/invitation.rb` is now a generic class for invitations. WebconfInvitation and EventInvitation
implement more specific stuff

refs #1216
Invite button opening modal + some translations

refs #1216
Refactor common behavior of invitations a little more

refs #1216
Merge branch 'branch-v2' into branch-v2-event-invitations

Conflicts:
	Gemfile.lock
	app/views/mweb_events/events/_registration.html.haml
	app/views/mweb_events/participants/index.html.haml
	config/initializers/mweb_events.rb
	config/locales/en/_mweb_events.yml
	config/locales/pt-br/_mweb_events.yml
Fix js bindings in the modal to invite people to an event

refs #1216
Improve the button to invite people to events a little

To look better together with the register button and in the list of participants
as well.

refs #1216
Merge pull request #25 from mconf/branch-v2-event-invitations

Add invitations to events, refs #1216