Knife command-line refactor plus addl features

* Refactored all knife commands to use the syntax:

    ```knife vault <sub-command>```

* Modified knife vault create to allow popping an editor window when values, json_file or file are unspecified on the command-line.

* Added knife vault edit.

* Changed all the banners to say:

    ```knife vault <sub-command> VAULT ITEM VALUES (options)```

  Instead of listing all the options out.  This is more consistent with most other knife plugins I've used.

Please let me know what you think.  I know it's a lot, but I've been thinking about tackling this for a while and finally got motivated last night/this morning :)

Happy to revise my commits or squash if it's easier as a single commit.