event-flick: Add necessary fixes as recommended by @ericf in
https://github.com/yui/yui3/pull/1323

* Improve `processArgs()` method
* Add code comments all-round
* Separate out `_isValidFlick()` into `_isValidFlick()` and
`_isValidAxis()`. The `flick*` events over-write the latter.
event-flick: update unit tests to work with changes in 5161f19