Refactor the loading of the JSON files

Hey Emils,

Thanks again for your recent work in adding test coverage and an updated algorithm to the referer-parser codebase.  Inspired by your changes, I refactored the way the JSON files were loaded a little.  I was expressly looking to achieve three things with these changes.

1. To diminish the reliance on module-level globals.
2. To match some of the other languages ability to load alternative referers.json files.
3. To pave the way for automated testing and Python 3 support.

I'd really appreciate your feedback on these changes (or the library in general).  If they look good to you, could you merge this PR with your fork and I'll pull them into snowplow/referer-parser and deploy them to PyPI?