Fixing replacing of \ to / on windows. String.replace('\\', '/') was replacing only the first occurrence.

see http://davidwalsh.name/javascript-replace
Fixing an issue when we pass "/" as the path argument when calling _add and _itemAt
adding rmdir
adding unlink
updating stat times in rmdir() and unlin() as per https://github.com/eldargab/node-fake-fs/pull/1#issuecomment-12680984
small refactoring on rmdir() and unlink()
adding rmdir() and unlink() to the methods that will patch fs
Merge pull request #1 from tgeorgiev/master