Get local charm icons from the Juju API.
Merge pull request #228 from frankban/local-icons

QA:

Check the local charm icons are correctly displayed
when using a real env.
- juju quickstart an environment;
- switch to this branch:
  `juju set juju-gui juju-gui-source="https://github.com/frankban/juju-gui.git local-icons"`
- wait for the GUI to be ready;
- deploy local charms including an icon:
  you should see the icons are correctly displayed in the
  service blocks and inspector header;
- deploy a local charm not including an icon:
  you should see a broken image: this will be fixed
  in a follow up branch;
- deploy a charm from the store: ensure the usual icon is
  still properly displayed both in the service block and
  inspector header;
- destroy the environment.

Ensure the fallback icon is used whe deploying local
charms in sandbox mode.
- `make devel`
- deploy local charms:
  you should see the fallback icons are correctly 
  displayed in the service blocks and inspector header;
- deploy a charm from the store: ensure the usual icon is
  still properly displayed both in the service block and
  inspector header.

Done, thank you!