Documentation update.

1. Renamed ceres.tex to ceres-solver.tex
2. Updated Solver::Options docs to reflect the recent changes.
3. Updated .gitignore to ignore ceres-solver.pdf

Change-Id: Iea19f8ff5fa1638a498422c8be5ed2e6da2950c9
diff --git a/.gitignore b/.gitignore
index 7935785..c1c6c08 100644
--- a/.gitignore
+++ b/.gitignore
@@ -13,3 +13,4 @@
 *.log
 *.synctex.gz
 ceres.pdf
+ceres-solver.pdf