Improved support for worker threads.

- Single processing mode now has full support for parallelization.
- Batch processing mode lines can now queue multiple entries in the
queue (similar to single mode).
- Disabling worker threads in batch mode lines ensures priority
processing.
Better support for ComicInfo meta-information

ComicInfo structure is now available as a separate set of classes,
implementing full serialization and deserialization support. This is
required for future addition of broken images management and recovery
system.
Improved code elegance of ComicInfo
Broken page placeholder and error tracking.
Binary release.