Merge pull request #86 from cbhaley/master

Improve quality of images manipulated by calibre. In particular improve ...
newdb: Fix passing in QImage to set_cover not working
Merge branch 'master' of github.com:kovidgoyal/calibre
Cover grid: Respect the double click on library view tweak
AZW3 Output: Dont choke on invalid links in the input document
Update FAQ on Kobo brokenness
Clear custom number columns to undefined when space is pressed