Merged and squashed mp_meta_keys

Squashed commit of the following:

commit b7f584975316eedeca89d468503e700b68327ebf
Author: Michał Podlecki <m.podlecki@op.pl>
Date:   Wed Jan 8 11:30:51 2014 +0100

    Admin can only create a new meta key

commit 1aa80d64ba748d284d1467fd01304b4e1e9bf7e4
Author: Michał Podlecki <m.podlecki@op.pl>
Date:   Wed Jan 8 09:27:47 2014 +0100

    Admin is able to add & edit meta keys