add Domino.find! method which waits for matching node

When working with asynchronous Capybara drivers, it may be necessary
to wait for dominos to appear eventually. The `find!` method uses
Capybara's `find` instead of `all` to wait for asynchronously created
nodes.

Capybara raises informative error messages if no match is found. This
prevents hard to debug "undefined method of nil" errors when trying to
interact with an expected unique Domino on the page.

* Added Domino.find!
* Added tests for matching, non-matching and missing selector cases
* Added section to README discussing usage with JS enabled Capybara
  drivers

Regarding test suite speed: The default Capybara driver does not support 
waiting. So the test which tries to find a non-existing node does not lag.