removing precompiled headers from makefile
Merge branch 'develop' into feature/issue-222-remove_precompiled_headers

Conflicts:
	make/os_mac
updated clean target to remove all artifiacts generated from .stan files in any subdirectory of src/test
Merge pull request #317 from stan-dev/feature/issue-209-clean-test-models

Fixes #209.
Merge pull request #316 from stan-dev/feature/issue-222-remove_precompiled_headers

Fixes #222. Removes precompiled headers from builds.
Merge branch 'develop' into feature/issue-185-command-line-handle-spaces