Add object.get(property_name, fallback_value) method

This method allows to lookup a property value and fall back to a
given value if the property does not exist, therefor allowing
things like

    result = object.get('some-list-property', [])
    for item in result:
        print item

instead of

    result = object.properties.get('some-list-property', [])
    if isinstance(value, consonant.store.properties.ListProperty):
        result = result.value
    for item in result:
        print item
Merge branch 'jannispohlmann/add-property-get-with-fallback-value'

Signed-off-by: Jannis Pohlmann <jannis.pohlmann@codethink.co.uk>
Reviewed-by: Adam Coldrick <adam.coldrick@codethink.co.uk>