Allow 0 num_workers to be passed in.

This turns off forking for production.
Eliminate a whole bunch of duplication in Gitpan.pm.

Merged import_from_distnames() and import_from_backpan_dists().
Put all the dist selection options inside Gitpan.pm for easier testing.
Move the distribution selection code into its own method.

Much easier to test.
With no search options, import all releases since the last complete run.
Avoid a deprecated warning from DBIC.
Make the timestamp file more descriptive.

It doesn't contain the name of the release, just the timestamp.
Log if we're in update mode, since when, and how many.
Merge pull request #117 from evalEmpire/issue/109

Update only what has changed since the last full run.