Added "destroy" method to allow for easy clean up of initialized popups.

[andrejpavlovic]

Implemented 'destroy' method as mentioned in #40.

Removes wrapper and background from DOM.
Removes global click handler.

$('#my_popup').popup('destroy');

[hlecorche]

@andrejpavlovic Your branch has conflicts
@vladimirsiljkovic What is your opinion on this pull request ? The idea is interesting.

[jmalfait]

@andrejpavlovic Thx for the solution!