Make childLookup accept *fuse.Context

A simple change that should be non-breaking given that it's additive

This then allows the `Context` to be passed to `GetAttr` as shown in the diffs for `fuse/nodefs/fsconnector.go`

Also updated the `all.bash` but I'm not clear on a couple of things:

1. How the script in its previous incarnation worked (I'm using Go 1.2.1)
2. Whether the tests currently work - my attempts at running them blew up on the `-race` invocation. Is there a CI running somewhere I can cross reference?

Happy to make amends etc as you see fit. I'm just starting out with Go...