Cronserver updates

A few small updates:
* Have cronserver load settings, and get frequency directly from settings (rather than hard-coding).  Partially addresses issue #199
* 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))