Added Waiter.wait_until!, ElementProxy#wait_until_visible, ElementProxy#wait_until_hidden

Added:
- Standardizing on "wait_until" instead of "wait_for"; the latter remains for backwards compatibility.
- ```Waiter.wait_until!``` - Just like Waiter.wait_until (no bang), but raises ```WaitTimeoutError``` if the condition is not met.
- ```ElementProxy#wait_until_visible```
- ```ElementProxy#wait_until_hidden```