Add previous button

[natelowry]

If it's on the first stop in the ride, it will hide the button.

Not tested with timers.

[redtail-rachel]

Thank you so much for this fix!!

[libinvbabu]

Works Perfectly! Thank you.

[robinfhu]

I was about to switch to another guided tour library, because joyride did not have a previous button. One of the reasons I like this library is that it does not depend on other libraries except jquery. Very lightweight.

Your pull request works and saved me a ton of work. Thanks!