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.
@mwmiller Probably shouldn't trigger with just "time" since we already have W|A for that:

![60](https://cloud.githubusercontent.com/assets/81969/4033908/bcc6d83e-2c80-11e4-9464-594e218b42c9.PNG)