Move wait_spec to watirspec

The wait_spec is being moved to watirspec - see watir/watirspec#40
Improved timeout message and add WhenPresentDecorator#present? and #respond_to?

For compliance with the watirspec/waitspec, the following changes were
made:

1. The element wait methods now include the element specifier in the
timeout message.
2. The WhenPresentDecorator now has #present? and #respond_to?