New select options, language, two column display and datetime fix

Select options now support term_select and posttype_select, and taxonomy_select now also support a simple NULL value if the user does not want to select anything... equally a short description has been added for usability reasons.

Language support has been added a few places, and to increase usability even further, I have found it convenient to add the option of displaying metaboxes in two columns. These must be added for each two fields in each new row:
'display' => 'half_first'
'display' => 'half_second'

Lastly, _datetime fields cleared itself after updating a post, so this was fixed as well.