Captcha

This changeset introduces recaptcha in registration form.

![screenshot from 2014-08-24 19 40 23](https://cloud.githubusercontent.com/assets/231601/4023392/8068f93e-2b8c-11e4-86b5-dfc8c9ac6d26.png)

However, I'm not quite sure whether we need to show captcha in edit mode, so it's disabled in edit mode.

![screenshot from 2014-08-24 19 40 37](https://cloud.githubusercontent.com/assets/231601/4023394/a1ed5866-2b8c-11e4-8fb9-b8bfd7953f47.png)