Moved saving in tab delimited formats from data.table to data.io
Add a function for faster saving of tab files
Improve the speed of save_tab_fast
Change ContinousVariable to property, left to_val use _out_format string for formatting
Add a doc string to Orange.data.io.save_tab_delimited
Merge pull request #99 from janezd/faster_save

Faster saving into tab-delimited files (#67)