Add license headers to the source files.

Change-Id: Iaf92cd1a3a8f1f65310a9df9e1631f68a9ef6f54
Reviewed-on: http://review.couchbase.org/42805
Tested-by: Aliaksey Artamonau <aliaksiej.artamonau@gmail.com>
Reviewed-by: John Liang <johliang00@gmail.com>