Integrate bestselling books feed
Refactor

Change Utils class to package object to be more idiomatic.
Integrate categorised bestselling books feeds

There are now multiple feeds that can be targeted by keyword.
Make some book properties optional
Fix loading bug
Refactor
Merge branch 'master' into kc-books

Conflicts:
	common/app/conf/switches.scala
Merge pull request #2912 from guardian/kc-books

Commercial: Integrate Bestsellers Feeds