Book details panel: Show custom column values for numeric columns even when the value is zero

Merge branch 'master' of https://github.com/cbhaley/calibre
Add context menu entries to clear metadata edit spinboxes for date and number fields
version 2.7
Update The New Yorker
E-book viewer: Add an option to prevent tapping on the page from turning pages.
Add the ability to fetch the user categories for a book to proxy metadata. Add a formatter function to use it.