Do not prevent input/textarea focus on down event
Fix a bug that prevented endOnly restriction

Would happen if inertia was not triggered and there was no endOnly snap
or snap was previously locked and unchanged.
Implement "parent" restriction

interact(element).restrict{
    drag: 'parent'
});

The rect of the element's parent is used as the restriction rect.
Implement "parent" origin

Involved adding element argument as target._element might not be the
element currently being interacted with (eg. durring drop check and
resolution).
Add "self" restrict and origin

Similar to "parent" but to the current element being interacted with
rather tnan it's parent element.
Merge branch 'bugfix-allow-input-textarea-focus'

Closes #8