0.6.3

This is mainly to close issue #28. The scratch field has been moved to ojph::local::codestream, and therefore it gets created and destructed with ojph::local::codestream.
Thanks to Dan for reporting this bug.
I think it is fair to say that Aaron in pull request #17 was alluding to this problem.
Thank you guys.

This also fixes a bug that was inadvertently introduced in the earlier commit, in not initializing of num_comp_bytes.