initial implementation of multifields

here's an initial implementation of multifield functionality which is essentially a grouping of fields which are bundled together and saved as a single array. there's still quite a bit to do:

* add support for more field types
* better adding/deleting UI
* minor css tweaks
* saving works but is a bit funky with the order in which it saves the groupings - there's also a php notice that needs to be fixed
* only text based field types are supported, other field types need to be tested and workarounds added one by one for the other field types - it's likely that some field types like wysywig will never be supported
* no sorbability for now

see #68
Merge branch 'master' of github.com:Automattic/custom-metadata