Fix access to config for non-sinatra scripts

Scripts such as bin/bulk_load which aren't running under sinatra don't
have the settings global variable available.  These have been broken
by a recent change which made the Index class access the settings
global.

Instead, inject the settings into Index via the constructor.  This in
turn requires corresponding updates to IndexGroup and SearchServer.
Merge pull request #235 from alphagov/fix_non_sinatra_scripts

Fix access to config for non-sinatra scripts