Clear _actions after performing action_chains

This will make possible to reuse `ActionChains` object after perform  for other actions wihout creating a new object.

case:
```
ac = ActionChains(driver)
ac.move_to_element(some_element).click().perform()
# ac._actions contains moving and clicking
ac.send_keys(some_keys).perform()
# ac._actions contains moving, clicking and sending keys, but I expect, that it contain only sending keys action.
```

Another solution is to add a new method `clear` witch will be clear self._actions list.