Add support for jemalloc

See http://www.canonware.com/jemalloc/

Change-Id: I1b5617b1534863f88f223675ccc7dc7fdc02f917
Reviewed-on: http://review.couchbase.org/37444
Reviewed-by: Dave Rigby <daver@couchbase.com>
Tested-by: Trond Norbye <trond.norbye@gmail.com>