Forces scheduler_worker.worker_stats to be JSON string

In gluon/scheduler we attempt to send a Storage() variable to the worker_stats field, which gets converted to a string during the INSERT.

For (at least) PostgreSQL JSON fields, this isn't valid:

```
2014-09-12 05:28:26,421 - web2py.scheduler.bastille-9.local#29597 - send_heartbeat scheduler ERROR - Error retrieving status: invalid input syntax for type json
LINE 1: ...heartbeat) VALUES ('ACTIVE','2014-09-12 05:28:26','<Storage ...
                                                             ^
DETAIL:  Token "<" is invalid.
CONTEXT:  JSON data, line 1: <...
```

This PR uses the simplejson dumps/loads functions to convert between Storage() and JSON field content. It needs further review to see if it will break other things down the line.