Performance improvements:
- Add an option to booklists to not check for duplicates if the device knows there aren't any. Eliminates N squared lookup times.
- Make the wireless device driver use the above option.
- Change a comprehension to use a generator instead of a returned list of keys.

FWIW: found these because a CC user has 18,000 books on his device!
Performance improvement again: don't change an existing method's signature.
Merge branch 'master' of https://github.com/cbhaley/calibre