Add "end time" field

We needed an "end time" field for our events in gigpress, as did others: 

* http://wordpress.org/support/topic/plugin-gigpress-end-times-on-gigs
* http://wordpress.org/support/topic/end-time-functionality
* https://getsatisfaction.com/gigpress/topics/add_end_time_to_show
* http://gigpress.com/blog/2008/07/version-134/#comment-145

So I decided to add that field:

![screen shot 2014-07-04 at 11 57 07](https://cloud.githubusercontent.com/assets/1552287/3480489/975f201c-0361-11e4-98cc-8d2ded392be6.png)

Haven't really done a lot of WP development so I might've overlooked something.

Thoughts?