Add 'fill_in' text field helper

Created a helper for setting text in text fields in the style of Capybara's "fill_in".

Usage example:

  fill_in 'Email address', :with => 'myfake@...'
  fill_in 'First name', :with => 'Dan'

Updated the built-in step "When /^I type "([^\"]*)" into the "([^\"]*)" text field$/" to use this helper.