Stop using 'jet' colour map by default

I've been meaning to post this before, and [this post](http://www.climate-lab-book.ac.uk/2014/end-of-the-rainbow/) just reminded me of it. Matplotlib has been [taking its time](https://github.com/matplotlib/matplotlib/issues/875), but they have to come up with a default colour map for a wide variety of cases. Since we have so few plots, I don't see why we can't choose a sane default for them ourselves. (And waiting for matplotlib will leave out all the users with older versions anyway.)

So far, I have found the following plots that could use fixing:
 - [ ] Tutorial:
  - [ ] [9 Beamforming Analysis](http://docs.obspy.org/tutorial/code_snippets/beamforming_fk_analysis.html)
   - The first scatter plot is default; second polar plot is hot_r.
  - [x] [11 Plotting Spectrograms](http://docs.obspy.org/tutorial/code_snippets/plotting_spectrograms.html)
   - Uses default.
   - Changed to gist_heat_r (though hot_r could maybe work as well). I'm actually able to see some information from 0s to 80s clearer than when it's all blue.
![plotting_spectrograms](https://cloud.githubusercontent.com/assets/302469/5115689/816169ce-7011-11e4-9c0a-71b5883f2567.png)
  - [ ] [23 Hierarchical Clustering](http://docs.obspy.org/tutorial/code_snippets/hierarchical_clustering.html)
   - Uses default.
   - I also don't like that the dissimilarity matrix doesn't match the colours on the dendrogram, though I don't know how easy it is to match them.
  - [ ] [24 Visualizing Probabilistic Power Spectral Density](http://docs.obspy.org/tutorial/code_snippets/probabilistic_power_spectral_density.html)
   - This seems to be a made-up colour map, though it seems pretty similar to rainbow.
  - [ ] [25 Array Response Function](http://docs.obspy.org/tutorial/code_snippets/array_response_function.html)
   - Very obviously the default, and it doesn't look good.
  - [ ] [26 Continuous Wavelet Transform](http://docs.obspy.org/tutorial/code_snippets/continuous_wavelet_transform.html)
   - All plots use the default.
  - [ ] [27 Time Frequency Misfit](http://docs.obspy.org/tutorial/code_snippets/time_frequency_misfit.html#plot-the-time-frequency-representation)
   - Tfr seems to be a different colour map from the rest.
 - [ ] [Catalog plots](http://docs.obspy.org/master/packages/autogen/obspy.core.event.Catalog.plot.html)
   - The examples use depth-colouring, but colouring by date will use the default colour map.

This PR is not nearly done, so suggestions are welcome.