Add mimetype to sicp.epub to comply with OCF spec

See section 4 of the OCF 1.0 specification: http://old.idpf.org/ocf/ocf1.0/download/ocf10.htm#_Ref134878301 .
Generate a smaller epub file

* Use max compression
* Don't generate directory entries
* Don't generate extended attributes
Dynamically generate src/mimetype

The uncompressed version was being overwritten by a compressed version
by the recursive second stage zip run when generating sicp.epub.
Add a `src/mimetype` target, exclude it when building the .epub instead of doing a create/remove dance.