Add an 'autoHide' option (same as osx behaviour)

Hi,

I added an option to disable the autohide feature. As I think this plugin is really useful even without that. Sometimes you don't want to have to mouse over an area to know that this is scrollable (when content is not obviously "cutted"). Or even if you don't have a mouse as input device you will not see the scrollbar (tv? touch devices? etc.).

You can test the option in the demo file with this:

`$('.demo1').TrackpadScrollEmulator({ 'autoHide':false });`

Notice that I didn't included the minified version of the plugin.
Readme is updated with option usage.

Thanks for your plugin that I really appreciate.

I would like to add more features/examples like scrollbar customization and mobile support. Tell me what you think.