updating display of error code in run_command()
updated run_command() to return an error code; updated all tests that use run_command()
from Michael: feature/fix_model_test_fixture
adding error_codes
changing quotes to <> for includes
adding tests that fail
adding src/test/gm/arguments/test_model to gitignore list
Updated src/test/gm/command_test.cpp and src/test/gm/arguments/argument_parser_test.cpp to pass
updating model_test_fixture to run test-models
working on argument_configuration_test
updating unit / model tests
Fix behavior of remove_duplicates when the value of an argument is empty
Merge branch 'feature/issue-202' of github.com:stan-dev/stan into feature/issue-202
updating model tests
Revert to displaying help when no arguments are given
updating run_command
Fixed argument_parser_test to account for argv requiring that the first element be the executable name
fixed test so it's windows safe
Merge pull request #207 from stan-dev/feature/issue-202

Fixes #202.

All tests pass.