Add invalid login message

This PR lets us display an invalid login message to the user when their login credentials are not accepted by the server. It also resets the cursor to the email field to allow the user to start typing again, if they so please. The invalid login message disappears on change, keyup, paste, or click events on either the email or password fields.

I used position absolute here as well to get the message on the side of the login fields. Feel free to let me know if there's a better way to do so!

Here's a screenshot of what the message looks like:
![invalid login](https://cloud.githubusercontent.com/assets/5490475/3723866/2bf8e1ee-1678-11e4-89ec-5a5db1b56408.png)

Checking that the message is visible is okay, but I believe it works without the if statement.  $.hide() should hide no matter what.