Added world_readable? and world_writable.
Values returned by this are platform dependent, Unix 777 has been chosen.
Merge branch 'master' of https://github.com/defunkt/fakefs into world_readable
0777 octal in decimal is 511. Change world_readable and world_writeable to return 511
Update tests for world_readable
Merge remote-tracking branch 'dekz/world_readable'
Use octal to make it clearer what the value returned means.