zpm: update and correct `create_project`

The `create_project` was old and wrong. Previously, it assumed the
creation of a handful of special directories. Now, all we need to do is
create a default `zar.json` file in the target directory.

TODO: I took my best guess at what the default zar.json contents should
be. This needs revision and more documentation.
test-requirements.txt: add `mock` dependency
test_zpm: add tests for zpm.create_project
commands: `new` command -- catch and print errors

If, for example, the target `zar.json` file already exists, catch the
exception and print a nice error message for the user (instead of the
entire stack trace).

TODO: Instead of using RuntimeError, it might be better to use a custom
exception class, so that we don't accidentally mask other unexpected
RuntimeErrors.
commands: updated `new` command doc

It now reflects the current implementation, which creates a default
`zar.json` in the target directory--unless the file already exists.