Change event start_time and end_time to datetimes

Events could not be multiday because they had a date
field. This removes the date field and make start_time
and end_time datetimes.

* Changes start_time and end_time on events to datetime
* Removes date from events
* Adds validates_timeliness gem for datetime validations
* Creates view partial for an event form
Refactor events

Trying to make the code friendlier to future
developers.

*Clean up tests in event_spec
*Rename user methods in event so it is clear
 they are returning hashes
*Rewrite feature spec for RSVPing to an event
Remove event_organizers table

This table was really just a subset of event_rsvps and was
causing duplicate records if a user had an rsvp to an event
and then was made an organizer.

*Drop event_organizers table
*Add organizer boolean column to event_rsvp
*Eager load event_rsvps in events_controller#show
*Add methods to make a user an event organizer
Merge pull request #85 from steven-ferguson/private-events

Event Refactor