Ebookmaker

- set "huge_tree" attribute for the xml parser to keep big files from blocking a build

- fixed bugs in rst->tex conversion
- class = 'medium' extra '}'
- removed '%' in noindent

- fixed bug exposed by setting cover with a command line argument

- refined embedded media error message to include referrer
- added external link warning

- Ebookmaker has been downloading and including in epubs embedded media from arbitrary websites, for example, images. This has caused build errors when a remote site goes away. ebookmaker has command line parameters that allow include and exclude urls that have governed document files; these same rules now apply to media files.

In general, PG books should not embed media from other sites.

Support docutils-0.8-style :class: language-<code>.