add taxonomy_multi_select field to the readme

see #51 and see #14
Updates select2 to version 3.4.1

New version of select2 fixes several problems I ran into by adding a
custom "display_callback".
Adds filters for _field_types_that_are_read_only and _field_types_that_support_multifield
Fixes error in multifields when cloning an item which is not at the end
Merge branch 'master' of github.com:Automattic/custom-metadata
Merge branch 'master' of git://github.com/Schreiber-und-Freunde/custom-metadata into Schreiber-und-Freunde-master
minor fixes to pull request #81

* coding standards adjustments
* remove some unused code

fixes #81, fixes #82
props PhilippSchreiber