Changing basefile to use path-platform's split.

Currently manta-sync will file.substr(localdir.length) leaving a basefile like `'\foo.txt'`, then later when you have the posix join you get a path like `'/mantauser/stor/\foo.txt'`

This change chops off the leading `/ or \` 
is this the same as calling `path.basename(file)`? if so, this won't work.