zpm: group imports

First block: stdlib
Second block: 3rd party
Third block: project (zpm) modules
setup.py: add `six` and `prettytable` to deps
commands: fix a spelling mistake in command help
zpm: add utils for building exec summary table
test_zpm: add tests for exec summary util funcs
Add `--summary` option to `deploy` and `execute`

This adds an option to display an execution summary when executing a
zapp remotely with `zpm deploy --execute` or `zpm execute`.
Merge pull request #182 from larsbutler/summary-table

Add execution summary table