UnixTime: subsume Epoch.pm functions.

There's no need to have a different Goodie to handle the "no remainder"
query case, so we'll just combine both into this single Goodie.

Also makes the goodie slightly more resilient against bad inputs by
`eval`ing all the way up through the DateTime creation and string
gathering.
Merge pull request #606 from mwmiller/epic_epoch

UnixTime: subsume Epoch.pm functions.