When copying books from library A to library B, check that B contains the necessary custom columns. If not, tell the user and allow them to create any non-conflicting columns. Inform them about conflicting columns.
Allow a device to determine the compression quality setting for thumbnail generation
Commit the other two files
Show a warning when copy to library is use don incompatible libs

Show a warning when attempting to copy books between libraries that do
no have the same set of custom columns. Fixes #1225484 [New feature: copy books between libraries with all custom metadata](https://bugs.launchpad.net/calibre/+bug/1225484)

Merge branch 'master' of https://github.com/cbhaley/calibre
Make default thumbnail quality 75