Fixes #19 (list of slices not shown in output of 'hanlon' command)

This pull request fixes the issue where the slice names were not output by the CLI script when a "hanlon" command was typed with no arguments (or when an unrecognized slice is typed after the "hanlon" command). Without the changes in this pull request, the following was output on the command-line:
```bash
$ hanlon
hanlon - DEVELOPMENT
Usage:
    hanlon [slice name] [command argument] [command argument]...

Switches
    -v, --verbose                    Enables verbose object printing
    -d, --debug                      Enables printing proper Ruby stacktrace
    -n, --no-color                   Disables console color. Useful for script wrapping.
    -V, --version                    Display the version of Hanlon
    -h, --help                       Display this screen

Available slices

$ 
```
With the changes in this new pull request, the hanlon script returns to the same behavior that it had before the big refactoring performed by @sankarvema shortly before Hanlon was released. This is what the new output on the command-line looks like:
```bash
$ hanlon
hanlon - DEVELOPMENT
Usage:
    hanlon [slice name] [command argument] [command argument]...

Switches
    -v, --verbose                    Enables verbose object printing
    -d, --debug                      Enables printing proper Ruby stacktrace
    -n, --no-color                   Disables console color. Useful for script wrapping.
    -V, --version                    Display the version of Hanlon
    -h, --help                       Display this screen

Available slices
	[active_model] [broker] [image] [model] [node] [policy]
	[tag]
$ 
```