Fixing bug with stopHidden option.

The logic around the 'stopHidden' option was reversed — if stopHidden is false, we don't want it to stop when it's hidden, thus _isVisible() should return true even though it's not, since we want the animation to continue. It's a bit confusing semantically, which could be improved by changing some function names...
Merge pull request #4 from jmwohl/patch-1

Fixing bug with stopHidden option.
I know some functions names are a little bit confusing, i´ll will going to review the names on the next version.
Thanks!