Paralellize execution of subcommands and make them chainable
Paralellize execution of subcommands and make them chainable

This commit changes the CLI interface of kraken substantially from
mostly separate subcommands to the definition of a chain of processing
steps (binarization -> segmentation -> recognition) operating on a list
of input/output file tuples. Each processing step is entered into a
queue with one or more worker processes executing them asynchronously
until all results are calculated. This approach improves recognition
speed substantially on multi-core systems.

mittagessen/kraken,mittagessen/kraken,mittagessen/kraken,QuLogic/ocropy,QuLogic/ocropy,mittagessen/kraken