Do not guess variable types for sql tables by default.
type_hints is an Orange domain.
Adapted tests for SqlTable guess_values.
Class value indexes should not be hardcoded as variables can be
recycled.
Merge pull request #108 from markotoplak/sqlopen

SqlTable: feature value guessing option, type_hints are domains.
Constant time boolean evaluation of SqlTables.
Add option in Sql Table widget to enable domain guessing.
Refactor (cached) __len__ for SqlTables.
Merge remote-tracking branch 'orange3/master'