Install Python library from PyPi

[bfirsh]

Instead of bodging the code in, it would be neater if we installed Cog using pip from PyPi.

This would require a few things:

• Publish Cog Python package on PyPi on release Automatically publish Python package on PyPi on release #146
• Install correct version in Dockerfile
• Keep current method of injecting Python library for Cog development

[victor9000]

@bfirsh In the meantime, are there instructions for building cog from source? I'm absolutely blocked by the current install method of downloading a binary using sudo and making it executable.