Improved credit card expiration entry experience

### Why?

The jQuery masked input plugin breaks form auto-fill tools like 1Password,
making it harder to enter credit info. By shifting to separate inputs
for expiry month and year, autofill works. This has an added benefit of
mapping more directly to the data required by payment processing
entities, requiring less intermediate code to parse input. Adding an HTML5
placeholder demonstrates the correct formatting for expiration dates,
accomplishing the goal of the masked input plugin.

### Changes:

- Switched to two fields for card exiration date
- Added HTML5 placeholers to expiration inputs
- Removed jQuery mask plugin (no longer used)
- Added validation on expiration date
- Updated tests and styling

![screenshot of credit card fields](http://cl.ly/MPek/content.png)

**Note:** I wasn't able to get the web framework set up on my machine (at
this time) which means I haven't confirmed the tests are working.
Hopefully this work will at least be a good start.