load_translation_table: load translation strings from enabled plugins
update translation utils:
po2php: add "if function_exists" around plural select function
run_xgettext: add option to extract strings from addon folder
update strings, add plural select function definition check
run_xgettext: check for dir before run xgettext
update util/messages.po
move load_traslation_table after db setup
add check for false returned by query in load_translation_table
Merge pull request #631 from fabrixxm/addonlangs

Per addon translation