Replace os.path (etc.) with twisted.python.filepath

[arch_o_median]

The filepath file system interface offers a single consistent object oriented interface for the various file system operations we need. Initially we just wanted to know if a file was the descendant of some directory, but this seemingly trivial question is not easily answered with the traditional python file system libraries (e.g. shutil, os.path, the open builtin, ... etc). Moreover these functions are not to be found in a single place, and lack a consistent interface. Additionally we think filepath is unique in offering atomic overwrite, (through setContent). So this ticket is to replace all the older ways of manipulating the filesystem with shiny new FilePath.