core: Add rebar_lock_deps plugin to freeze the dependencies of release branches

Squashed commit of the following:

commit 3b493ff9732adb3ba1fc152821a1b0ef7e69eead
Author: Arjan Scherpenisse <arjan@scherpenisse.net>
Date:   Wed Oct 1 11:48:36 2014 +0200

    core: Add rebar.config.lock

commit e9597db453c31823d9771f9649ae020d3888d25a
Author: Arjan Scherpenisse <arjan@scherpenisse.net>
Date:   Wed Oct 1 09:20:27 2014 +0200

    Add lock-deps plugin. Fails with an undef error though.