BK-794 Aloha plugin for word count

Adds menu item called "Word count" to menu Tools.
After clicking the dialog appears with following info: word count in chapter, character count and character+spaces count.
Merge remote-tracking branch 'sf_booktype/2.0' into ticket_BK-794

Conflicts:
	lib/booktype/apps/edit/templates/edit/aloha_header.html
BK-802 Aloha plugin for word count
Merge branch 'ticket_BK-794' into 2.0