Added Remove{Next,NextUpdate,All,AllUpdate} and tests; modified old tests.

Added:
- RemoveNext - Remove the next scheduled event, leaving the clock untouched.
- RemoveNextUpdate - Same as RemoveNext, but update the clock.
- RemoveAll - Remove all scheduled events, leaving the clock untouched.
- RemoveAllUpdate - Same as RemoveAll, but update the clock to the latest
  scheduled time.
Added testing code for all of these functions.

Additionally, added more documentation to old tests, and fixed a bug
where machines which fixed the return value of time.Now() could cause
tests to fail (all tests used fixed values, and never call time.Now()).