added new funciton inv to stan
added new function inv_sqrt
added new function inv_square
Added log1m_exp: math, gradients, tests and doc
Added log_diff_exp: math, gradients, tests and doc
Improvement to ordered_logistic distribution: now it uses log1m_exp(.) instead of log1m(exp(.)).
Merge branch 'develop' into feature/adding_inv_functions
added inv, inv_sqrt, inv_square to doc
fixed inv, inv_sqrt, inv_square functions and added boundary tests
updating make instructions for generating src/stan/models/model_header.hpp.d
refined boundary tests for inv, inv_sqrt, inv_square
Tweak output to mirror old notation, update tests
removed redundant operations in inv_sqrt, inv_square, pow
Merge branch 'develop' into feature/adding_inv_functions
Tweak output to mirror old notation, update tests
Properly handle appended samples
Merge branch 'feature/output_tweaks' of github.com:stan-dev/stan into feature/output_tweaks

Conflicts:
	src/stan/io/stan_csv_reader.hpp
Merge pull request #75 from randommm/feature/log1m

log1m_exp and log_diff_exp
Use BFGS for point estimates by default.
Introduce improved convergence criteria for BFGS optimizer.