Add a 'fill_in' helper for setting text in a text field.

Inspired by Capybara: https://github.com/jnicklas/capybara/blob/master/lib/capybara/node/actions.rb
Simplifying boolean condition in 'fill_in' helper.
Merge pull request #196 from cyrusinnovation/fill_in_text_helper

Add 'fill_in' text field helper