add Bode plot to Response object
changelog
Response plot: add line cross for instrument sensitivity
response plot: changes to call parameters

- more intuitive for users to be able to specify sampling rate +
  frequency step (i.e. min/max/step of frequencies) in plot (rather than
  t_samp and nfft)
- make it able to provide two axes for the plotting instead of making a
  new figure (will be used in plotting multiple channels in
  Station/Network/Inventory)
response plot for Channel class
Response/Channel plot: make it possible to pass in label string for
legend
Station plot: response plot working but might need some more work..
Station.plot(): only leave option to plot response

makes it simpler and clearer, for a map location plot it is probably
easier to nest the station into a network and use that for plotting
Station.plot(): take location codes into account
Merge remote-tracking branch 'upstream/master' into station_plots_master
station class plots: add example plots to docs
station type plots: fix plotting grid (just `grid()` toggles grid state)
Station.plot(): remove start/end stage options...

makes no sense in many cases because responses for channels might be
pretty different and matching a range of stages across channels
usually won't make much sense.
refactor event plot into reusable basemap/scatter plot routine (for
inventory plots)
Merge remote-tracking branch 'upstream/master' into event_plot_refactor
just warn on missing basemap (to make rest of imaging test suite run properly)
Two new FDSN tests.

Tests the created URLS. Fixing some minor bugs along the
way.
Adjusting test cases once again...
Custom per-service endpoints can now be specified.

Also added a short test.
changelog