Bliss tango server addons and fixes

Fixes:

• default value for sessions property
• missing argument in motor_group_state
• group motion should be asynchronous
• ignore hidden files when searching for tango server classes
• ignore objects that will start with BlissAxisManager server instead of Bliss

New features:

• get/set settings commands
• axis_device_names attribute
• reload_config and apply_config command