Merge pull request #79 from elliot42/master

Auto-discover location of `pip`
Auto-discover location of `pip`

When `pip` is installed manually it shows up in _not_ `/usr/bin`, but
rather `/usr/local/bin`. Switch to using auto-discovery to find the
appropriate executable.
Enable retrieving build artifacts from CircleCI

This commit adds utility functions for downloading build
artifacts from CircleCI.  This allows anybody to leverage
CircleCI's handy build automation workflows.
Merge pull request #81 from elliot42/master

Enable retrieving build artifacts from CircleCI
Avoid dict comprehensions

They are not compatible with Python 2.6.
Specify positional params in format'd strings

...so that Python 2.6 won't balk at them.
Avoid `subprocess.check_output`

...because it's not available with Python 2.6.