fixed broken variable - added duration feature

[sixfngers]

using this in an upcoming project. fixed issue and added a single duration capability feature that made it a simpler usage for my project without taking away any functionality from the previous implementation

[robho]

I think I'd prefer if a single integer is passed in as $durations in the case where the same duration should be used for all frames. Ie use is_integer($durations) and is_array($durations) to determine what user wants. Supplying an array of size 1 looks strange to me.

Maybe also change the name of $durations since it's no longer an array of several durations so it could be confusing to the user.

Also, don't forget to update the documentation :-)