add examples for taxonomy (multi)select fields

fixes #70
fixes #51
see #44
add taxonomy_multi_select field to the readme

see #51 and see #14
Marking select as a field type that supports multifield
Fixes an error when display_callback is an array

display_callback did not work when using a callback like
array(&$this, 'my_callback')
Merge pull request #77 from Schreiber-und-Freunde/master

Fixes an error when display_callback is an array
Merge pull request #73 from rinatkhaziev/master

Marking select as a field type that supports multifield
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
Adjustments to the README based on features added
Merged upstream/master.