Add AAAS journals to subplot command

[bradyrx]

@lukelbd, this PR adds the AAAS journal specs to the proplot.subplots() command. It also has a few minor fixes:

• Move the journal specs table to the header and change name to JOURNAL_SPECS following PEP (https://www.python.org/dev/peps/pep-0008/#constants).
• Fix try/except clause in subplots() that was causing certain journal specs to break.
• Add pytest for all journal specs as a template for future testing.
• Update changelog and add capability in docs to link from changelog to PR/issue threads.

This is a great simple example of pytest's power. I created the test then ran pytest and found consistent errors in certain subsets of the dictionary (TypeError vs. ValueError). Once you get pytest implemented into Travis CI, this will run automatically on PRs and then you have no risk of importing broken code. At least broken journal spec code for now.

[bradyrx]

Feel free to review directly if you want anything changed. Or you can pull it down and edit, push, then merge. You need to add a travis CI webhook, and .travis file, etc. for the tests to run on a PR like this.

[bradyrx]

FYI, this is a solid template for travis: https://github.com/bradyrx/climpred/blob/master/.travis.yml. I may be biased though.

Then you just need to activate your repo here (https://travis-ci.org/) and add the webhook. Then it will always run on every PR, ensuring that the package can be built and that all testing passes.

[lukelbd]

Looks good to go. Thanks for the tips. Didn't know about the PEP8 recommendation re: constants. I have a bunch of constants throughout the module, decided to also make them all caps (c2ae600) and gotta admit it's nice to know at-a-glance where each function is using a global variable.

[bradyrx]

Didn't know about the PEP8 recommendation re: constants.

These are the kind of things that come up through open source code review via PRs. I learned that one from a collaborator on climpred.

FYI, @lukelbd, the ext_links feature allows for this (clickable PR links in a changelog): https://climpred.readthedocs.io/en/latest/changelog.html

So eventually you might want to host that on your docs.