Changed keyActions from attribute to property.
Added setter for the focusedCell attribute.
Added keyIntoHeaders attribute to determine whether to
navigate into the headers.
Dropped the default call to preventDefault on any recognized
key.  Now, each function/listener must call it, if required.
Added user guide documentation for datatable-keynav.
Added tests to navigate into headers or not.
Changed the tests that use keyActions since it is now a property.
Dropped the 'groups' YUI configuration entry.