Better duration notation for non-default steps, eg: 1 hr 15 mins

I noticed that if the `step` setting was set to anything other than 30 or 60, the notation for the duration of an option could be confusing. For example, a 15 minute interval could result in *(1.8 hrs)* which makes no real sense.

Now, using a 30 minute step will keep current notation of *(1.5 hrs)*, but setting it to something else will give you something like *(1 hr 45 mins)*.

I've added the class name `.ui-timepicker-step-x` (where x is the `step` value) to make sure the wrapper is wide enough for this longer notation.

![screenshot 2014-05-22 12 05 03](https://cloud.githubusercontent.com/assets/59516/3052282/97243834-e198-11e3-8ca3-fcef8c5c7fbb.png)

Oh, and I haven't minified the JS here just yet.