Additional Erlang support: prove executable; rebar options; ability to have rebar targets not part of "ALL"

Change-Id: I280cf12b924410957759e3ea7dc7597ceee16553
Reviewed-on: http://review.couchbase.org/35100
Reviewed-by: Trond Norbye <trond.norbye@gmail.com>
Tested-by: Trond Norbye <trond.norbye@gmail.com>