Test CSS3 gradient generator with Jasmine

Add tests around the CSS3 gradient generator

See https://github.com/dgalarza/CSS3Factory/pull/3 for test framework set up.

CC @acostanzo 
It's the reason the thoughbot guides specify to: 
> Avoid its, let, let!, specify, before, and subject in RSpec.

https://github.com/thoughtbot/guides/tree/master/best-practices#testing