0.15.0

Closed issues:

• Main program does not exit cleanly #153

Merged pull requests:

• Fix threading gateway persistence scheduling #154 (MartinHjelmare)
• Clean up get_const #151 (MartinHjelmare)
• Remove leftover old style string formatting #150 (MartinHjelmare)
• Add unique id for gateway #149 (MartinHjelmare)
• Use pytest for tests #148 (MartinHjelmare)