Merge pull request #181 from larsbutler/execute-logging

execute: enable the --log-level option
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
commands: add `--summary` option to zapp execution
zpm: add utils for building exec summary table
test_zpm: add tests for exec summary util funcs
Add exec summary option to `deploy` and `execute`