fix bug 809523: API access to document summary

* ?summary option on document URLs to serve just the summary (combines
  with ?raw & etc)

* $children doc view now includes summaries, for page.subpages in
  kumascript

* $json doc vies now includes summary, for wiki.getPage in kumascript

* Added option to get_seo_summary to strip / not strip markup

* get_seo_summary will now first look for a "Summary" section
Merge pull request #872 from lmorchard/809523-page-summary-api

fix bug 809523: API access to document summary