added stubs for all of the cholesky corr factors, w/o tests
add factor_U and factor_L functions to transform to CPCs
Merge branch 'feature/corr-cholesky-type' of https://github.com/stan-dev/stan into feature/corr-cholesky-type
fix factor_L()
add test of factor_L
intermediate state of Cholesky to allow pull of Ben's code
merged transform.hpp
Switch to maintaining the inverse Hessian in BFGS.
Restructure BFGS options for reuse in LBFGS.
Factor out the Quasi-Newton update into its own representation to prepare for L-BFGS.
Implementation of L-BFGS.
Enable L-BFGS in command.hpp
Added basic unit test for L-BFGS.
Merge branch 'develop' into feature/lbfgs

Conflicts:
	src/stan/common/command.hpp
	src/test/CmdStan/optimization_output_test.cpp
Added vectorization to multi_normal_log
Vectorization of multi_normal, multi_normal_prec and multi_normal_cholesky + tests
vectorization of multi_student_t_log
Changed "response variable" to "random variable" to be consistent with the Stan
added doc for vectorized multi_normal and added subsection in vectorization chapter to describe how it works
added doc for vectorized multi_normal_prec, multi_normal_cholesky, multi_student_t