Improved validation

Replaced HTML 5 validation with Parsley.js validation. Fixes #32 and #67. The styling is inspired by Firefox Accounts:

![screen shot 2014-07-09 at 8 43 24 pm](https://cloud.githubusercontent.com/assets/3095/3534231/6a8d836c-07e4-11e4-8ddb-19d23e89bccc.png)

@pdehaan r?