Expand API docs and add missing test coverage

[twm]

• Add missing modules to the API docs
• Fix various bits of broken reST syntax
• Add _Response.collect() method which was in the API docs, but not actually implemented!

To do:

• Clean up the index a little (the headers become a little redundant with these changes).
• Add a test for _Response.collect(), unless reviewers think that it should be removed from the docs instead of added to the implementation.
• Add a tox rule to build the docs. They're probably so broken because there's no out-of-the-box way to check what they look like.

There is now a warning in the Sphinx build which I haven't managed to get rid of: sphinx-doc/sphinx#2549

[codecov-io]

Current coverage is 96.54% (diff: 100%)

Merging #150 into master will increase coverage by 0.22%

@@             master       #150   diff @@
==========================================
  Files            20         20          
  Lines          1713       1738    +25   
  Methods           0          0          
  Messages          0          0          
  Branches        152        154     +2   
==========================================
+ Hits           1650       1678    +28   
+ Misses           42         39     -3   
  Partials         21         21          

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[glyph]

I want to just merge this but it has pending conflicts :( can you address those, please, @twm?

[twm]

@glyph Okay, done.

[glyph]

Awesome. I was just helping out a Twisted user today and had to lament the fact that treq.testing did not show up in the API docs. Thanks a bunch!