Allow user data model to be configurable in the accounts API

Allows selection of a user defined userdata_model for the accounts API.

The following will need to be added to Accounts config

/*
 | --------------------------------------------------------------------------
 | Account Admin - Userdata model
 | --------------------------------------------------------------------------
 |
 | Sets the userdata model that the Accounts API should use. Leave empty
 | to use bitauth_userdata_model
 |
 */
$config['userdata_model_name'] = 'tps_bitauth_userdata_model';
$config['userdata_model_path'] = 'database/tps_bitauth_userdata_model';