HA integration: Dynamic expose

[farmio]

Description

• HA integration: add knx.exposure_register service allowing to add and remove ExposeSensor at runtime
• HA integration: move Expose classes to own module and create exposures after xknx.start() was called
• XKNX: add Device.shutdown
  • delete its callbacks
  • remove RemoteValues from StateUpdater
  • remove from Devices
• mitigate warnings in tests when calling __del__ when the loop is already closed

the intention behind this is the same as in #535

Type of change

• Bug fix (non-breaking change which fixes an issue)
• New feature (non-breaking change which adds functionality)
• Breaking change (fix or feature that would cause existing functionality to not work as expected)
• This change requires a documentation update

Checklist:

• I have performed a self-review of my own code
• The documentation has been adjusted accordingly
• The changes generate no new warnings
• Tests have been added that prove the fix is effective or that the feature works
• The changes are documented in the changelog
• The Homeassistant plugin has been adjusted in case of new config options

[coveralls]

Coverage decreased (-0.01%) to 94.851% when pulling b4339a9 on farmio:dynamic-expose into 725e787 on XKNX:master.