Combine get and list commands under single get command

[carolynvs]

This is the first step in switching our command structure to be more consistent with kubectl (see #25). I have not changed any implementations, so formatting cleanup, etc will be done in a later step.

• Moved list commands under the get command, e.g. svc-cat brokers list is now svc-cat get brokers.
• Removed the extra "catalog" subcommand. So svc-cat catalog classes list is now svc-cat get classes.
• Standardized variable names for easier copy/pasting towards victory.
• Made all the help text consistent.
• Added examples to the help text.

Closes #27

[carolynvs]

The cla bot magically became happy all by itself so... this is totally ready for review now. 😂

[arschles]

@carolynvs thanks for addressing my comments. LGTM, merging