move some code in xknx.io module

[farmio]

Description

• Moved RequestResponse and its subclasses to a new package: xknx.io.request_response
  It is only imported from tunnel.py (apart from tests and some example code) so I think this should be ok.
  I kept mixing up tunneling.py and tunnel.py in xknx.io

• Moved ConnectionConfig to a new module xknx.io.connection

Fixes # (issue)

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 increased (+0.0008%) to 94.714% when pulling 553c69b on farmio:io-refactor into 583ac0c on XKNX:main.