Starting/ending page functions, improved metadata

Added two functions, returning starting page of an article and ending
page of an article. Improved metadata generation using those functions.
Improved start/end page metadata

Added additional conditions, so starting/ending page metadata tags won't
be included if the getStartingPage and getEndingPage functions don't
return any data.
Merge pull request #120 from Looted/master

Starting/ending page functions, improved metadata