System check.

This adds a GET system/check endpoint to report the overall system status.  This includes showing log locations and how much space is available on the logs' drive(s).  This adds some tracking and reporting of cherrypy threads.

This adds a PUT system/check endpoint to perform a consistancy check.  This checks to make sure all items have valid sizes and valid folderIds.  This should eventually be extended to check other models (see TODO list).

Fixed the plugin enable/disable test (it was expecting toggling one plugin would only affect one plugin, but because plugins can have dependencies, this isn't necessarily true).

Order json output by default.

Convert to using psutil for disk space on filesystem assetstores.  This should make the capacity work more generically (not just on linux).

Increased item test coverage.

This addresses issue #577 and starts to add features for issue #496.