Allow multiple icons for column icons

Column icons: Allow the use of multiple icons with column icon rules.
You can now have column icon rules display multiple icons in a single
column, side by side. There are two ways to do this, either specify
multiple icons when creating the rule, or create multiple rules that
match the same book and specify the icon type to be 'composed' for every
rule. See Preferences->Look & Feel->Column icons for details.

Merge branch 'master' of https://github.com/cbhaley/calibre
pep8
Add support for new cover handling in Kobo Aura

The new Kobo Aura has firmware version 2.9.0. The main change needed for
the driver is that cover images are stored in a different place.

Version number is set to 2.9.1 to handle the later release for other
Kobo devices.
Speed up the qhash implementation
Fix switching to an empty virtual library not blanking the book details panel
...
Fix regression that broke html metadata

HTML Input: Fix a regression that broke processing of HTML files
that contain meta tags with dc: namespaced attribute values.
Fix formatter function "count" to return zero for an empty list
Make an uncomposed icon rule use all the icons from previous composed icon rules. Lets you express things like "compose icon a if XXX, compose icon b if YYY, icon c if ZZZ, icon d if WWW". This will result in categories icons a and b if their conditions match, then appended with one of: c if its condition matches, d if its condition matches, or nothing.