methods for score at given time
A method to add seconds to a clock time.

I've added this so that time ranges can be specified with more
flexibility.
Overhauled @ersherr's play scoring with the following:

- plays_through_time has been generalized to plays_range,
  which returns a slice of plays, in sequence, given any
  range of time.
- score_in_plays heuristically adds missing XP data.
- score_at_time was left in tact, but the time value should
  now be an instance of the Clock class.

Close #17.