Improve "help" feedback

[menegazzo]

1. Asking for "help" should return only the script names.

   > help
   I can help you with the following matters:
   - hello
   - futurama-quotes
   - jenkins
   

   For detailed information about any script:

   > help hello
   Here are the commands I understand:
   
   'hey|hello|hi': Send greetings to Bender
   'help': Beg for help
   'shutdown|quit|exit': Shutdown Bender handling. Careful!
   

2. Add support for "context": some commands are not intended to be public (like shutdown). Maybe respond hook should receive an extra argument context then asking for help will return only public commands.

   > help
   All public commands available...
   > admin help
   All commands intended for administrators
   > foo_context help
   ...