Optional implicit waits

[tmertens]

This resolves the optional implicit waits portion of Issue #41. Simply add the following to your test code (e.g. spec_helper.rb or some other place) to enable implicit waits for the has_ and has_no_ methods in SitePrism.

SitePrism.configure do |config|
  config.use_implicit_waits = true
end

Note I used the name "use_implicit_waits" instead of "existence_check_should_wait" because it seemed more generic in case it is applied to other queries.

[tmertens]

Added documentation.

[natritmeyer]

I'll get to this next week :)

[tmertens]

👍

[natritmeyer]

Thanks @tmertens :)