Fix Entry#contained?

Some applications, like BibDesk set the title and the booktitle to the same value. This would lead to a false positive when calling #contained?
Skip #booktitle if title == booktitle
Fix #year

Output was "2014-feb". Is now "2014-2"
Remove bibo:created and bibo:issued from #year

They do not exist. (Links to Dublin Core)
Use DC.issued only in #year

bibo says about DC.issued: "Used to describe the issue date of a bibliographic resource"
whereas it says about DC.created "Used to describe the creation date of a bibliographic item".
Add #date_added
Remove some bibtex fields from the fallback by default
Bibliography#to_rdf

Every entry was included multiple times in the graph returned, as each conversion of an entry did not know of each other when it created the children and parent

The code style follows Entry#to_rdf