Allow users to submit time and distance as set parameters

[nskins]

There are various exercises that are better expressed in terms of time and/or distance than weight and reps. For instance, the user might want to record that she ran 3.1 miles in 20:07 (20 minutes and 7 seconds).

We should first choose which unit of measurement to use for distances. I think I'd actually prefer meters over miles because there's a lot of running exercises on the track that are better measured in meters (400m, 800m, etc.). Likewise, swimming and rowing exercises are more often measured in meters.

The time should be expressed in the format hh:mm:ss. The UI could be constructed in a way such that the user wouldn't have to manually enter the colons.