refactor with some minor added features

refactor items:
- fix requires/paths
- use mojito-cli api, env calcs, exports

new:
- check is-an-app
- check app depends on mojito
- check mojito is installed or specified
  - can specify --lib path/to/mojito
- do proper post-listen error handling
  - check EADDRINUSE
  - note these need yahoo/mojito#1150 to land, noop otherwise
- invoke cb when done, use real non-zero exit codes on error