Adding [docs only] and [build docs] custom tags

[bsipocz]

Minor clean up of [travis skip], and addition of two new custom tags to the top level script.

This address all ideas in and thus close #188

[bsipocz]

@Cadair @astrofrog - I fear this is something that cannot really be tested within this repo, if you up to it, please give it a go. (E.g. without usage I've totally forgotten within a month that [skip travis] is already a thing...)

[bsipocz]

I'll send out an e-mail to astropy-dev and the maintainers list about this, but also tagging a few people who I know are keen to keep the CI footprint minimal (though it would be great to educate the less keen people about the merits of it). @mhvk @hamogu @cdeil @MSeifert04 @eteq @astrofrog @pllim @larrybradley @Cadair @mwcraig

All the new custom tags work like the prerelease builds, and set the commit status to pass. Obviously it doesn't mean that the tests are passing. We can assume that the author has good reasons for the skip, but maintainers should be aware of this, too to avoid after merge surprises.

So, now we have [skip travis] and [travis skip] to skip travis. And more importantly have [docs only] or [build docs] that will skip all jobs except the docs building ones.

These tags cannot be tested in this repo, but I run a few tests elsewhere. While it works as expected, please look out for issues that are related to this.

[mhvk]

Fantastic! We should be sure to document it also in the development guide.

[pllim]

That's a great addition. Thanks, @bsipocz !

[mwcraig]

This is excellent !!

[larrybradley]

This is great. Thanks, @bsipocz!

[kain88-de]

@bsipocz I'm not sure but this might have broken the cron job for MDAnalysis. For us it is now always run. Even during a normal PR. Below is an example job with DEBUG=True

Cron Job
Whole Travis Build

[bsipocz]

@kain88-de - Indeed, this PR cleans up a bit of the logic, and does all checking for cancelling builds before any install happens. I suggest to use common setup_conda.py (as shown in the README) rather than the OS specific setup files.

[kain88-de]

Ah yeah thank you. I overlooked that we use the specific init script.