Associated models with a has_one relation is not created by default, it's now up to the user to explicitly create it.
Minor refactoring.
Fixed spec for nested form. Had to activate testing through Webkit to make it work.
Edited Travis config to run specs with xvfb.
Replaced capybara-webkit with poltergeist to get better support for Travis. It was not without cons though, had to set some values in the spec with jquery since poltergeist gave me all sorts of trouble.
Removed config.use_transactional_fixtures = false since it wasn't necessary
Changed DatabaseCleaner settings not to mess with mongoid.
Some improvement of specs
Fix spec failures
Merge branch 'optional-has-one-rails-3.x' into rails-3.x. Backport of #1632