Added KVB Profile

[1Maxnet1]

As wished in #21 I implemented support for the KVB Profile. The format of the HAFAS-API is a bit different than the default so I had to override the Journey and the Journeys classes for the profile.
I also added some tests, at least for the requests and docs following the existing docs.

Please let me know any comments regarding the PR

[leona-ya]

I fixed a bug that came first up in this profile (I hope this was okay). Otherwise looks good :3

[1Maxnet1]

I fixed a bug that came first up in this profile (I hope this was okay). Otherwise looks good :3

Thanks, sure this is okay 👍

[akloeckner]

When browsing through the tests, I found that one test uses the VSN profile still:

pyhafas/tests/kvb/request/journeys_test.py

Line 8 in e7bb642

client = HafasClient(VSNProfile())

[1Maxnet1]

When browsing through the tests, I found that one test uses the VSN profile still:

pyhafas/tests/kvb/request/journeys_test.py

Line 8 in e7bb642

client = HafasClient(VSNProfile())

Good catch, thanks. @leona-ya I would create a new PR as follow-up tomorrow morning, if you don't mind :)