More flexible pip requirements.txt layout

I've re-arranged requirements.pip into a directory that contains a base list of packages (requirements/base.txt) which gets inherited by dev.txt and prod.txt.  The purpose is to modularize the set of requirements that are required for different deployment environments (development, staging, ci, qa, testing, production, etc.)  

For example, we do not want to deploy some django and/or python debugging packages to production servers.  For example: django debug toolbar, ipdb, coverage, pep8, cx_oracle, etc.

I replaced the original requirements.pip with a top level requirements.txt that defaults to the production settings.  (It seems that the ".pip" extension is being phased out, in favor of ".txt")