added option to ignore empty or small objects

adding `min_size:1` or any other integer to `device` definition in job description
will instruct objectquery middleware to ignore objects of the size smaller than `min_size`
it can be good for various error channels, for example placing `min_size:1` on `stderr` and saving `stderr` as object
will produce the object only in case where `stderr` was not empty
Merge pull request #94 from pkit/empty_objects

added option to ignore empty or small objects