Merge pull request #1928 from minrk/nodb NoDB is now the default backend for storing Hub results in IPython clusters. This prevents the use of delayed retrieval of results from the Hub, but means that the default use will have significantly reduced memory footprint (and some natural use patterns won't run into memory exhaustion as easily as before). All these advanced features remain available with any of the 'real' storage backends (dictdb, mongodb and sqlitedb). A small bug discovered along the way when using HubResults for execute requests is also fixed and tested.