add readthedocs hosted documentation

[edaniszewski]

this PR largely just adds in documentation that will be hosted on readthedocs.

the slate API docs are still here, but will no longer be hosted. I still need to figure out how to either host them on github pages while also supporting readthedocs, or how to link them directly into the readthedocs files. I'll save that issue for a different day. Once checked in, these docs should appear at http://synse-server.readthedocs.io/en/latest/

I've temporarily enabled rtd builds on this branch so these docs can be read there (probably easier than reading through all the .rst files here): http://synse-server.readthedocs.io/en/rtd-docs/

Any feedback, suggestions, comments, etc is welcome as always.

I'll squash all of these commits down when it comes time to merge.

fixes #120

[codecov-io]

Codecov Report

Merging #122 into master will not change coverage.
The diff coverage is n/a.

@@           Coverage Diff           @@
##           master     #122   +/-   ##
=======================================
  Coverage   88.98%   88.98%           
=======================================
  Files          40       40           
  Lines        1080     1080           
=======================================
  Hits          961      961           
  Misses        119      119

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