Move data out of config.yml, and genericise data handling.

Changes to allow the back-end database to either be contained in a YAML file, or eventually SQL (not yet implemented).

 - Most functionality removed from the main plugin module: farmed out to other modules.
 - New database interface, plus YAML (complete) and SQL (stub) implementatinos
 - New config manager module, plus new default config file
    - config file allows selection of YAML/SQL, and SQL settings/YAML file.

Done some basic testing on a local Minecraft server:
  - checked all commands do not result in a crash, and function as expected
  - manually inspected new .yml file to check data sane
  - tested with missing .yml files to check base generation works.