Kicking off new bug trackers (for a project) properly

This patch adds a new API option 'include_empty' to customs/api.py, for kicking off new trackers/projects when also specifying the 'just_stale' switch. So far, new projects had an empty bug list and wouldn't be included in the results...and therefore wouldn't get scraped for new bugs initially.

I've spent about 5 minutes staring at this code and I'm still a bit mystified as to why this does what you claim, @dirkbaechle. Mind providing a walkthrough?