Fix bug in gb-show

If a gradebook file had records like the following, gb-show would wet the
bed:

    Student,Name:95

The problem was the missing space after ':'.

This version of gb-show does *not* assume a space follows the colon.
Instead, it cleanly handles either case: space or no space.

In a nutshell, find the colon, use it to split line into name versus rest,
then trim the rest (so that any possible spaces will be removed). Easy,
once you see it.