Helloworld fix

By default, the hello_world example fails to run:

    $PROJECT_ROOT/cubes/examples/hello_world> slicer serve slicer.ini

    2013-12-10 15:03:50,356 WARNING no backend specified, using 'sql'
    2013-12-10 15:03:50,357 WARNING no section [workspace] found in slicer config, using empty options
    2013-12-10 15:03:50,357 INFO using backend 'sql'
    ERROR: No URL or engine specified in options, provide at least one

These issues are related to the default values in the slicer.ini. Adding a backend value to the [server] block removes a distracting debug message and changing the "datastore" config block to be named "workspace" resolves the fatal error when trying to run the demo dev server.