Add a captcha to /contact

[nickom]

http://churnalism.sunlightfoundation.com/contact/

To combat the sudden increase in spam bots:

[nickom]

So all of these spammy emails have this in the message: [/url], [link=

I was able to create an email filter to screen these but probably good to still add a captcha when you have a chance.

[dvogel]

Added django-humanity. Thanks @jcarbaugh!

[jcarbaugh]

You're welcome @dvogel!