Add a #to_s method on the exhibit class.

Fixes #349 
This is primarily so that MailForm can properly print out the current exhibit in the email body.

We may want to drop the current exhibit from the email body in the future, but for now this will address the current issue.

![screen shot 2014-02-26 at 1 25 44 pm](https://f.cloud.github.com/assets/96776/2275926/6d7b5d30-9f2d-11e3-8f48-7f43db5e627f.png)