app: introduce object state with ObjectCtrl

This splits the ContainerCtrl into two, one controller for handling
listing pseudo-directories (paths ending with a trailing slash) and
another for listing single objects (other paths).

Before, drilling down in the hiearchy of pseudo-directories would
eventually take you to a page with a "listing" of a single object.
This happened when the prefix excluded all other objects. This page is
now empty -- we will use it to show object specific metadata.
swift: add headObject method

Despite what the documentation says, there is no expectHEAD method in
version 1.2.25 of angular-mocks.
object: show metadata (fixes #99)
app: rename path to prefix

This better matches how we use this with Swift.