extract methods on an Extractor object should take a block

[jeremybmerrill]

So rather than doing like this

extractor = Tabula::Extraction::ObjectExtractor.new(pdf_filename, [1])).extract
extractor.each &:whatever
extractor.close!

we can do something like

extractor = Tabula::Extraction::ObjectExtractor.new(pdf_filename, [1])).extract do |extractor|
    extractor.each &:whatever
end

which is far more rubyish

obviously, might be worth holding off on doing this until we merge in tabula-java and tabula-extractor is just the ruby bindings.