Allow mentors to specify recurring appointments

(Fulfills #65 per @redsquirrel and @alexbotsford)

Simple interface has been added to the "Offer to Mentor" screen:

![screenshot 2014-03-18 10 22 05](https://f.cloud.github.com/assets/202/2449493/9c179666-aeb1-11e3-9799-0705cbe6b7b0.png)

You can choose "every week", "every 2 weeks" and "every month" and specify up to 10 recurrences via the dropdown.

Once the user is activated you should see the recurrences appear on the front page:

![screenshot 2014-03-18 10 23 17](https://f.cloud.github.com/assets/202/2449525/e1a90052-aeb1-11e3-8f67-ee2b1c0da905.png)

And the user management screen:

![screenshot 2014-03-18 10 23 36](https://f.cloud.github.com/assets/202/2449530/e8dbcb34-aeb1-11e3-8dd0-51463e96033e.png)