Handle zero wait time passed to a waiting finder

Capybara 2.1+ supports a :wait option on all finders which can be used
to disable the waiting behaviour:

  has_text?("Home", wait: false)
  has_css?("ol li.options", wait: 0)

Fixing code which hits the timeout in this wait will often require less
thought and fewer changes than using a non waiting finder like
`all(...).any?` or `first`.

I also think it reads better and is more explicit.
Merge pull request #7 from jonleighton/allow_zero_wait

Handle zero wait time passed to a waiting finder