minor docstring fix (see #808)
Ported #807 to releases branch.

This nables reading INFO request responses from the IRIS ringserver.
Changelog
Removed future imports for releases branch.
Don't accept non-keyword arguments.

Trace.spectrogram() does not accept non-keyword arguments.
Merge pull request #816 from bmorg/fix_spectrogram

Minor Stream.spectrogram doc fix
Merge pull request #811 from bmorg/fix_807_for_releases

Fix #807 for releases branch
Merge branch 'releases'

@bmorg, maybe you can give this merge a quick check, merging in
things backported to releases is always a bit messy..

Conflicts:
	obspy/seedlink/client/seedlinkconnection.py
	obspy/seedlink/tests/test_slpacket.py