REST: Implement mci_get_version() and use it

• Implement mci_get_version() similar to mci_get_category()
• Move mci_get_category() from mc_issue_api.php to mc_api.php.
• Change mci_get_category() to return null instead of array( ‘category’ => null )
  in case of REST when category is not set.
• For SOAP if category is not set, return null from mci_get_category().