0.3.0: Merge pull request #15 from cloudify-cosmo/support-zip

• Added a --format flag allowing to also choose the zip format for the wagon. Default is tar.gz.
• Wagons now have the .wgn extension.
• It is no longer required to provide a package_name==package_version when creating from PyPI. using wagon create -s flask will create a wagon of the latest version of Flask found in PyPI.
• Arbitrary pip args can be passed to the create and install commands via the -a flag. e.g. wagon create -s flask -a '--retries 5'.
• To pack up internal requirement files, you can now simply provide the -r flag instead of -r . which will look for requirement files within the path/archive. To explicitly provide additional requirement files, you can use the additional pip args feature.