Give user an amplify button. Closes #121

Ready for review.

Create a migration for an amp table linking event and users. Also created controller for Amps to handle create and new amps.

Once a user has amplified an event, show the event as having been amplified.

Added controller, button to view

@southpolesteve - I pulled heavily from registrations for this. Not sure if I need the equivalent of an org_administration model as well? Not sure if the before_filters in the controller are needed.