forms: Add new regular expression validator

This allows administrators to define a regular expression to filter content entered into short answer fields by agents and endusers. With the new visibility feature (included), the regular expression input is only shown if the regular expression validator is selected.

##### Configuration selecting the new regular expression validator
![image](https://cloud.githubusercontent.com/assets/672074/4259084/1c8460a0-3ae3-11e4-937d-55dbf63eed5c.png)

##### Regular expression field disappears when a different validator is selected
![image](https://cloud.githubusercontent.com/assets/672074/4259089/3baca37a-3ae3-11e4-935e-a29300dc9ae8.png)

##### Text not matching the expression is considered invalid
![image](https://cloud.githubusercontent.com/assets/672074/4259091/550d2c04-3ae3-11e4-8b1f-32f7fcd00f15.png)