Add dbdiff utility to compare two couchstore files

dbdiff is a command line utility you may use to compare the
content in two different database files to ensure that they
have the same content.

Change-Id: I21360daaef0be0638e26c38a1c793e7681e629f6
Reviewed-on: http://review.couchbase.org/36570
Tested-by: buildbot <build@couchbase.com>
Reviewed-by: Trond Norbye <trond.norbye@gmail.com>