Cronserver updates

This now fully addresses issue #199:

* Have cronserver load settings, and get frequency directly from settings (rather than hard-coding)
* Make sure cronserver runs the same python that it was called with (important for testing purposes (machines with multiple python installs), but possibly in production as well)
* Add explicit path information to manage.py, to ensure that  cronserver calls runs manage.py from the same project as itself (irrespective of what directory it is launched from) (this doesn't look like an issue, but is more explicit and safer (doesn't depend on particular behaviors of subprocess or OS))

And now:
* Call cron directly from code (async, through subprocess), for all jobs except securesync
* Reduce frequency of cron server to once every 10 mins (by default, but still configurable), to take care of syncing

This branch now depends on #206 and #202