Added option to DTBook to ZedAI

New option:
"copy-external-resources", true by default.

From this request:
https://code.google.com/p/daisy-pipeline/issues/detail?id=329
DTBook-to-zedai fixes

Fixed MODS bug
(https://code.google.com/p/daisy-pipeline/issues/detail?id=328)
Cleaned up verbosity of output messages
(https://code.google.com/p/daisy-pipeline/issues/detail?id=343)