containers: allow selecting containers (fixes #50)

Later, we will add some actions for the selected containers.
RootCtrl: refactor allSelected and toggleAll funcs

Since $scope.containers and $scope.items is reassigned, we cannot just
bind the allSelected and toggleAll functions directly to them.
test: add e2e tests for selecting containers
ContainerCtrl: support selecting objects (fixes #50)
controllers: no containers/objects means no checked toggle all
Merge pull request #51 from mgeisler/checkboxes

Add a column of checkboxes