LUN-1371: minimized number of queries for sitemap
when trying to get the last modification date for a page
LUN-1371: * added check for plugins mod dates existence
since there might be pages with no plugins in them
LUN-1371: pages from sitemap should not be re-fetched each time
the paginator's objects_list is requested.
LUN-1371: removed unused method
Merge pull request #23 from pbs/LUN-1371

LUN-1371: minimized number of queries for sitemap