Writing docs and specs for spec-helper

I've started writing documentation for the goodies that `spec-helper.coffee` adds to all Atom specs. This addresses my comment in #3399.

@benogle, I've finally had time to work on this, but I just started so it's not ready to merge. I added specs for the custom matchers, as they prove to be useful in documenting the matchers, much like Jasmine documents [its own matchers][match].

Let me know if I'm on the right track with regards to organization and style. I'll be pushing some more commits soon.

[match]: http://jasmine.github.io/1.3/introduction.html#section-Included_Matchers
Did you mean `toShow`? I can make different PR that changes `toShow` to test for [every visible display-value][dv]. Otherwise, I'll just document `toShow` as is.

I'm just concerned that this change might break specs that are using this matcher already.

[dv]: https://developer.mozilla.org/en-US/docs/Web/CSS/display#Values