Fixed the `PageElement` wait methods and other stuff. Check description.
Fixed the `PageElement` wait methods and other stuff. Check description.

Renamed `wait_appear` to `wait_exists` and `wait_disappear` to
`wait_absent` on `PageElement`.

The `PageState` object no longer uses `appears` and uses `exists` and
instead of `disappears`, `absent` is used.  This is an effort to make
the syntax more explicit.

Wykleph/selenext