Adding loading model dialog to the dashboard.

Check for database connection before showing the dashboard. Once the database is
up, the modal dialog will disappear and all graphs will be loaded.

The tick charts and tables were not loading even after database has started
before because the Connection object initializes valid procedure names at the
beginning and never updates. So it will refuse to call user procedures even
though the database is up.
Merge pull request #4 from VoltDB/loading-dialog

Adding loading model dialog to the dashboard.