Add links on how to install pip

The most common problem, based on submissions to Reddit,
encounted by users is how to install PRAW. They don't
have pip installed and encounter difficulties installing
it. By adding links to tutorials detailing how to install
pip, the number of people having problems with installing
PRAW should decrease.
Minor documentation fixes

 * Add punctuation to the end of all lines, save those that
       are bullet point listings.
 * Add to CHANGES.rst that distinguish can also be used to
       distinguish as special.
 * Keep consistent spacing in CHANGES with 1 line above and
       below each release.
 * Removal of unnecessary empty lines.
Move get_liked and get_disliked to Redditor.

Redditors can make their likes and dislikes public. Having get_liked
and get_disliked on Redditor instead of LoggedInRedditor allows PRAW
to access this info.
Add (un)set_contest_mode to Submission
Fix bug that prevented configuring timeout setting
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