Make HMTL event listing view in CiviEvent more flexible

Currently, HTML event listing is available only as CiviCRM page, with entouring headers/footers, etc. It can be useful if we allow raw HTML output, that could be included in external pages, either as PHP function to be included in PHP mode in Drupal, or as JS function which will include HTML in the page. Advantage of the latter is that event listing could be included in any type of external page.