updating Stan to v2.3.0
adding CmdStan artifacts for v2.3.0
Merge pull request #20 from stan-dev/feature/issue-19-update-Stan-v2.3.0

Fixes #19. Tag as v2.3.0.
updating git ignore
updating built artifacts
Merge pull request #23 from stan-dev/feature/issue-19-update-Stan-v2.3.0

Feature/issue 19 update stan v2.3.0
moved Stan's executables to CmdStan
moved Stan's executables to CmdStan
Merge pull request #26 from stan-dev/feature/issue-25-move-executables-to-CmdStan

fixes #25. moved Stan's executables to CmdStan
updating stan
Merge branch 'develop' into feature/lbfgs

Conflicts:
	stan