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.
Fix a divide by zero error, more accurate level of detail computation.

Qt's levelOfDetailFromTransform seems to flush subnormals to zero.
Fixed template download url.
Merge remote-tracking branch 'orange3/master'