PAC usage / docopt inconsistent

[JKrag]

The docopt section (in core.cli_text) that defines the usage of pac is currently inconsistent/incorrect.

The main Usage: section defines to main usage patterns (apart from --help):
pac from ...
pac from-latest-tag ...

but the Options: section defines two switches called --from and --from-latest-tag.

Apart from the fact that this is confusing and inconsistent, it also means that pac currently accepts, without failure, input like:

./pac.rb from 55857d4 --from fb49307

or

./pac.rb from 55857d4 --from-latest-tag

According to @MadsNielsen the "correct" one is from as a command even if it seems a bit inconsistent with the --to switch.

Suggested solution:

• The docopt section should be changed to remove the two deprecated switches (--from and --from-latest-tag)
• The docopt should instead document the legal commands from and from-latest-tag in a separate section. Docopt allows this by just adding more sections. See fake git example for inspiration.
• While we are at it, the docopt should also document the rather important --to switch.
• The docopt should probably also contain at least a simple Examples section (see calculator example )

[buep]

That is a well described change you mention. It would be great to implement that next time we change something, or maybe before.

Relates a bit to #57