Move :wait to helper method to not create unneeded Query instances

This pull request makes changes into code added by https://github.com/jnicklas/capybara/pull/1111

Currently additional instances of Query are created unnecessarily in `assert_selector`, `assert_no_selector`, `has_text?`, `has_no_text?`. They are used for the sole purpose of getting `wait`.

I believe it's better to extract method to Helpers than to create additional instances of Query when they are not needed at all (especially in `has_text?` and `has_no_text?`)

Also I removed unneeded print in spec added by #1111