Minor internal improvements

 * Use alphabetical ordering for _get_section methods in Redditor.
 * Use requests.ok instead of magic number 200. Like in 7f0b6e04acb7f.
 * Flatten 6 if.. else for brevity and code clarity.
Add notes about clients voting.
Use new has_fetched attr instead of _populated

This will enable users to see whether the data for an object has
been fetched without using a private attribute.
Add Lazy loading page to documentation.
Switch to using "Nature" sphinx theme.

It's prettier. Has clearer headlines separating the content. It
includes a search for searching PRAW rather than RTD. The sidebar
also wraps lines, whereas in default it cuts off lines that are
too long.
Add note to CHANGES.rst about proper links on RTD

This note is not visible when looking at the changelog on RTD.
Deploy 2.1.7