Documentation theming

As came up in #820, we might want to investigate modifying the theme somewhat. To that end, I have re-generated the documentation with a variety of themes. You can see the results [here](http://qulogic.github.io/obspy/). Note: I had to remove the custom `layout.html` and `custom.css` because it was causing issues with some of the themes. So I also re-generated the default theme we're using and it might be slightly different from the usual ones.

I did not go through to ensure everything was laid out correctly. I was also lazy and didn't regenerate images, so some may not fit as they should.

From the builtin themes,
 * ~~[default](http://qulogic.github.io/obspy/default/) is starting to look a bit old, but, on the other hand, it does work...~~
 * ~~[sphinxdoc](http://qulogic.github.io/obspy/sphinxdoc/) looks pretty good.~~
 * ~~[scrolls](http://qulogic.github.io/obspy/scrolls/) is fixed width, and seems a bit small, so I'm thinking it's not too responsive.~~
 * ~~[agogo](http://qulogic.github.io/obspy/agogo/)  also looks good, except for the tables.~~
 * ~~[nature](http://qulogic.github.io/obspy/nature/) also looks good. Base font size is a tad small, but otherwise good.~~
 * ~~[pyramid](http://qulogic.github.io/obspy/pyramid/) also looks good. The function header bar could use a little tweaking.~~
 * ~~[haiku](http://qulogic.github.io/obspy/haiku/) is a bit small in places, but otherwise good.~~
 * ~~[traditional](http://qulogic.github.io/obspy/traditional/) looks, well, old.~~
 * ~~[epub](http://qulogic.github.io/obspy/epub/) is probably not a good choice.~~

Some external themes too:
 * ~~[better](http://qulogic.github.io/obspy/better/) seems a bit bare to me. The advantage is that this one works pretty nicely on mobile.~~
 * ~~[flask](http://qulogic.github.io/obspy/flask/) uses a bit of a large font, but otherwise okay. Also works nicely on mobile.~~
 * [bootstrap](http://qulogic.github.io/obspy/) is the one we're expecting to use...

- [ ] Fix untracked files in `misc/docs/source/packages/obspy.core.quakeml.*`
- [x] Share the top navigation bar between the docs, the tests, and the to-be-built proper homepage
    - [x] Download navbar and footer snippets during the doc build process and included as static HTML file in the template
    - [x] Add requisite CSS and JS
    - [x] Server version w/o downloading of snippets
- [x] Fix bootstrap CSS that's loaded multiple times
- [x] Move javascript to bottom and have the defer attribute in order to improve page loading
- [x] Use new logo
- [x] Add changelog
- [x] Update overview page for Bootstrap
- [x] Fix rendering of citations page
- [x] Do something with sidebar on index pages (currently just removed them, since they end up empty)
- [x] Gallery theming
- [ ] Remove local copy of directives?
    - [x] `plot_directive` is from matplotlib
    - [ ] `autosummary` is from sphinx