Return an error on unknown query types in the SQL driver
Correctly show the symbol in Q types String() method
Replace a panic in a test with a call to t.Error()
Log number of task instances when starting/finishing them
Call Task.executeTask in a goroutine

Otherwise there can't be concurrent instances of the same task,
so only a goroutine was used for the whole task previously.
Refactor gnd.la/tasks to avoid using gnd.la/log directly
Exit with an error when an undefined command is provided
Add query.Contains Q

Translates to a LIKE in SQL, produces an error in backends without
support for Contains (like GAE)