Amdin MetaTerms feature

Squashed commit of the following:

commit 5b4ff7f03dab4df8f092983b2dc0a8e2a3733436
Author: Michał Podlecki <m.podlecki@op.pl>
Date:   Thu Feb 27 09:54:05 2014 +0100

    add meta terms section in admin panel

commit 88e00c7e7311c3bac88bd5f812adeb90ce3f7bc6
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 d392aa8754fb6ec44d1c68746a63f9a224021beb
Author: Michał Podlecki <m.podlecki@op.pl>
Date:   Wed Jan 8 09:27:47 2014 +0100

    Admin is able to add & edit meta keys