Improvements on multifields

- Fixes the GUI of multifields and uses standard worpdress gui elements (fixes #71)
- Allows to delete the first item (in fact only deletes the values of the fields if there is only one item left, fixes #78)
- Allows other field types (fixes #79)
- Adds sortable functionality (fixes #80)

I don't know, how I can remove the commits 6ab1430 and 61164d2 from this pull request so please ignore them. Relevant are the commits on Feb 20, 2014.