Make RPM cope with an input epoch of zero.

Some RPMs from jpackage.org have their epoch set to 0, causing epoch to
raise an exception.
Actually, just cope with epoch being a number in general.

Further to checking if epoch is zero, just allow epoch through if it's a
number, as it should be.
Merge pull request #343 from barn/epoch_of_zero

Make RPM cope with an input epoch of zero.