Add Rename support to fuse.

Basic testing for source/dest existing and not existing, golang behavior
differs from POSIX eg os.Rename does not cause EEXIST if target exists.

Fixes bradfitz/docker#12

Docker-DCO-1.1-Signed-off-by: Paul Nasrat <pnasrat@gmail.com> (github: pnasrat)
Merge branch 'fuse-rename' of github.com:pnasrat/docker into fuse

Conflicts:
	vfuse/all_test.go
	vfuse/client/client.go
	vfuse/vfused/server.go