Adding 'BeagleBone for Secret Agents' to examples

Second chapter has a neat example for making a bridge with a bandwidth monitor
on a BeagleBoard.
Better support for multiple hidden services

Adds the following Controller methods for manipulating hidden services.
 * get_hidden_services_conf
 * set_hidden_services_conf
 * create_new_hidden_service
 * delete_hidden_service

Fixes issue #12533 where creating new hidden service would clobber the
configuration for existing services