Add theme hooks to buddylist_buddylisting_page

This very simple change adds two buddylist specific theme functions to allow a user to easily change the contents of table rendered on the buddylisting_page (?q=buddylist). The listing remains a table but these two hooks allow you to change the contents of the table header and to theme each row (1 row per buddy account) to your liking. The default theme functions result in a table that is identical to the current table output of 1.0.

The two theme functions are:

// Theme the header for the buddylisting table - must return an array of column header values - return array() if you don't want any header
function theme_buddylist_buddylisting_header() {
  return array(t('@buddy', buddylist_translation()), t('online'));
}

// Return the theme row for the buddylisting table - $account is the information of the current buddy being themed, $online is TRUE if the $account is currently online
// Must return an array of theme contents for each column specified for the table
function theme_buddylist_buddylisting_row($account, $online) {
  return array(theme('username', $account), theme('buddylist_online', $online));
}

Eventually, it would be nice to not be hardcoded to display as a table but for now, this small baby step provides a lot more external flexibility.