station: many small fixes

- flake8
- enable doctests
- parse networks in Inventory.get_contents()
- adjust docs module page
station: proper plugin handling for Inventory, add read_inventory to
obspy top-level namespace
station: implement missing pieces in response.py

some things are still not parsed correctly, e.g. some tags have
attributes with information that is not yet read (e.g. number attribute
of coefficients in responses or uncertainties associated with floats
etc.)
station: add (failing) test that reads/writes a stationxml file with
*all* fields (adjusted random generated full stationxml so that all
elements that appear in choices are in use somewhere)
changelog (alphabetical order)