tests/test_zpm: add tests for util functions

This adds tests for `_generate_job_desc` and `find_project_root` in the
`zpmlib.zpm` module.
zpm: add docstrings to some util funcs

This add docstrings to `_generate_job_desc` and `find_project_root`.
Merge pull request #114 from larsbutler/improve-test-coverage

Improve test coverage
zpm: abstract `_get_zerocloud_client`

This utility can be used elsewhere to handle the various auth args and
create a ZeroCloud client object.
tests/test_zpm: tests for _get_zerocloud_client
miniswift: add `download` method

This method can be used to download single files from a container,
similar to `swift download`.
Implement `zpm execute` command
tests: update cram tests to include `execute` cmd