parse ConnectResponse Telegrams with Error codes

[farmio]

from @uweswrtz in #272

Description

I've taken some changes form #272 to parse ConnectionResponse Telegrams with Error codes "correctly".
From my local tests nothing changes to current xknx behaviour with these changes, but the parsed error code can be used later in the RequestResponse() or Tunnel() class. I plan to do that in #443

Also added tests from #272 (rebasing this was too much trouble at this time) Thanks @uweswrtz !

closes #272

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 (-0.1%) to 94.647% when pulling b3e2298 on farmio:error-connect-response into ec278a8 on XKNX:master.