Check if keyboard is visible before animations

Check if keyboard is visible before performing animations on it after
UIGestureRecognizerStateEnded.
When the user swipes down too fast, the keyboard is no longer visible,
but jsq_handlePanGestureRecognizer:(UIPanGestureRecognizer *)pan still
tried to animate it.
Returning early instead of large conditional block

In response to jessesquires pointers in the pull request issue_213_fix.
Compare using <= 0.0f instead of == 0.0f

Slightly more robust, in case the keyboard is ever positioned even
lower than the height of contextView.
Improved keyboardViewIsHidden calculation
Merge pull request #231 from koenbud/issue_213_fix

issue_213_fix - Check if keyboard is visible before animations