bug fix; performance improvements

- move SZMAPI_ definitions to global context so they aren't evaluated
  every call
- inline bytes_to_int in one place for 10x performance improvement
- fix typo: strLen->iddLen
Added to_zip function.
Compute checksum more efficiently.
Merge pull request #2 from dmbaggett/master

tnefparse performance improvements and bug fixes; also new to_zip function for converting TNEF attachments into ZIPped ones