images: move rescaling from server/image to pkg/images

Fixes http://code.google.com/p/camlistore/issues/detail?id=94

Change-Id: Ifa73e0a3ccbbcaef31ae8870d39f63b8a90aad26
Merge "images: move rescaling from server/image to pkg/images"
fs: use schema.DirReader, rather than doing it by hand

Change-Id: I7e79621d621314ed1f50219f0cafc44b1b82ab1f
Merge branch 'master' of ssh://camlistore.org:29418/camlistore
fs: fallback to modtime of permanode if file's is unknown

Change-Id: I0b9c58dc6b226526eac4a6772e2125b396f3fc66