Convert documentation from LaTeX to Sphinx.

A live version of the doc can be found at

http://homes.cs.washington.edu/~sagarwal/ceres-solver/

As it stands, the documentation has better hyperlinking
and coverage than the latex documentation now.

Change-Id: I7ede3aa83b9b9ef25104caf331e5727b4f5beae5
16 files changed