Ftrack module refactor

[iLLiCiTiT]

Changes

• ftrack module should be possible to import and create all the time (Python 2/3 and with/without ftrack_api)
  • removed Python 3 syntax in code which can be used in Python 2 too
  • removed all imports in ftrack module __init__ file that will try to use ftrack_api module
• class FtrackServer is using file import function from pype.lib (moduke_from_path)
• imports from ftrack.lib or ftrack.ftrack_server must be explicit
• moved scripts from ftrack_server folder to scripts folder
• renamed folders with event handlers
  • actions -> event_handlers_user
  • events -> event_handlers_server
• replaced usage of CustomDBConnector with PypeMongoConnection
• abstracted custom ftrack session and updated with newer initialization code

[mkolar]

Tested in Nuke, Maya and Blender. Publish ok, actions ok, all seem to be fine