feat(rxAccountUsers): add switch user directive

rxAccountUsers provides the ability switch between users under the
same account.  This directive is specific to Rackspace.
feat(rxAccountUsers): Use nav isActive

Rather than checking for `/cloud/` in the URL, take advantage
of the `key: 'cloud'` we use in the nav, and check if that
nav item is currently active
Merge pull request #610 from rackerlabs/isCloudAPI

feat(rxAccountUsers): Use nav isActive
chore(rxApp): Dangling comma and more explicit description
chore(rxApp): More concise callback
fix(rxAccountUsers): Properly populate urls when outside of Cloud
chore(rxAccountUsers): User _.defaults instead of _.assign
chore(rxApp): Use new Encore API
Merge pull request #618 from rackerlabs/fix_cloud_urls

fix(rxAccountUsers): Properly populate urls when outside of Cloud
fix(rxAccountUsers): Broken test
fix(rxAccountUsers): Use  to prevent app reloads
Merge pull request #606 from rackerlabs/rx-account-users

feat(rxAccountUsers): add switch user directive