Check highlight option in mouseover() instead of highlight().

See issue #190.

This allow external methods to control the highlight status, just as it is possible to set isSelectable to false, and control the selection status from external methods.
Effectively, mouseover() and mouseout() no long call the highlight() and clearEffects() methods if the highlight option is false. Also, the highlight() method now highlights the area, regardless of the highlight option.

Note: I did not add the build output (dist/jquery.imagemapster.js and dist/jquery.imagemapster.min.js). Part of the src in the original repository has MS-DOS line endings, some UNIX line endings, and rake can't handle that, leading to spurious lines in the diff.