Added several filters and the ability to import group images as the post featured image.

The new filters allow you to customize the API to Custom Post Type mappings:

• add_filter( 'ccb_include_group_participants' ), whether or not to ask the API to include group participants in the response
• add_filter( 'ccb_get_groups_taxonomy_map' ), ability to customize the taxonomy mapping for groups
• add_filter( 'ccb_get_groups_custom_fields_map' ), ability to customize the custom fields mapping for groups
• add_filter( 'ccb_get_calendar_taxonomy_map' ), ability to customize the taxonomy mapping for events
• add_filter( 'ccb_get_calendar_custom_fields_map' ), ability to customize the custom fields mapping for events