admin: meta context with meta key definitions section and groups section

Squashed commit of the following:

commit ed25df156659ab8c6aa15851afd01467e9129c58
Author: Michał Podlecki <m.podlecki@op.pl>
Date:   Mon May 26 10:09:09 2014 +0200

    meta context with meta key definitions section

commit cb07a0a5573440fdef6d52b348ecb929fd3d9466
Author: Michał Podlecki <m.podlecki@op.pl>
Date:   Mon May 26 07:34:42 2014 +0200

    admin meta context groups section