Improve packaging process (including versioning and dependencies)

[NikolasOliveira]

This ticket spawned from a conversation here, which was regarding the packaging process. Work items/suggested improvements include:

• Improve documentation describing the packaging and publishing process
• Use a central version module and import it where it's needed, so that there is only one location to bump version numbers.
• Use a single requirements file that describes the necesarry dependencies for webdiff (load/parse this file in setup.py)
• Automate the packaging/publishing process (see twine)