Do not create empty filters.
Handle connection errors.
New code for computing conditional distributions.
Updated README.
Merge pull request #125 from astaric/handle-connection-errors

Handle connection errors.
Only warn about continuous vars in Sieve (like in Mosaic).
Use SQL data_download instead of setting .X and .Y.
Test continuous.
MajorityFitter returns ValueError when given Cont class.
MeanLearner predictor, widget and unit tests.