spritePath() -> filePath() and GraphicsConfig->ResourceConfig

- graphics.json renamed to resources.json
- graphics.json and board.json moved into a chespp folder.
- "chesspp" node removed from both json files
- GraphicsConfig class renamed to ResourceConfig
- spritePath(s) funcitons renamed to filePath(s)
- filePath(s) functions no longer append "chesspp" to the beginning of a path

resources.json is now used for any resource that requires a file path (not just sprites). 
Thus, GraphicsConfig class and spritePath(s) functions were promptly renamed. 

Fixes #67 and part of a discussion in #65