L-BFGS Documentation

Documentation for L-BFGS, and an example of training binary L2 logistic regression using L-BFGS.

Author: DB Tsai <dbtsai@alpinenow.com>

Closes #702 from dbtsai/dbtsai-lbfgs-doc and squashes the following commits:

0712215 [DB Tsai] Update
38fdfa1 [DB Tsai] Removed extra empty line
5745b64 [DB Tsai] Update again
e9e418e [DB Tsai] Update
7381521 [DB Tsai] L-BFGS Documentation

(cherry picked from commit 5c2275d6e4639946fd11ff6403338c8a9ade3d1e)
Signed-off-by: Reynold Xin <rxin@apache.org>