Multiple release-history sources

Currently there is no way to specify multiple urls for sources.  If a project is not found in a custom repository, a pm-download fails.  This patch adds the ability to have several sources in sequence.

--source is split using explode(',' $source), but still defaults to RELEASE_INFO_DEFAULT_URL
--source+ is split using explode(',' $source), and then prepended to --source
--source++ is split using explode(',' $source), and then appended to --source

Prepending is done before appending.