Refactor library into class files

[karlfl]

The library could benefit from a simple extract of the classes into their own .py files. This will provide for easier maintenance and future expansion of the library. It should be possible to do this in a way that doesn't break existing code.

Something like this...

1. adafruit_httpserver.py - remove/replace with the following folder
2. adafruit_httpserver (new folder)
   • __init__.py
   • httprequest.py
   • httpresponse.py
   • httpserver.py
   • httpstatus.py
   • mimetype.py

[dhalbert]

As an aside, see #17.