MB-100: Update .gitignore to include .DS_Store files

Change-Id: I6e06bcc9dbf17fcbd42feb71fc226756211c53c2
Reviewed-on: http://review.couchbase.org/27871
Reviewed-by: Trond Norbye <trond.norbye@gmail.com>
Tested-by: Michael Wiederhold <mike@couchbase.com>