Add configurable option to exit on warnings.

Signed-off-by: Bebe Peng <bpeng@pivotal.io>
Add docs for exit_on_warning option to README

Signed-off-by: Bebe Peng <bpeng@pivotal.io>
Fix errors on CI

- We were experiencing errors because we were not cleaning up tests that
  used the DisplayMessage.exit_on_warning! method

Signed-off-by: Bebe Peng <bpeng@pivotal.io>
Merge pull request #167 from gpleiss/feature/configurable_warnings

Configureable option for exiting on warnings