Remove current Entry#to_rdf
Refactor Entry#to_rdf by introducing Entry::RDFConverter

Every conversion is in its own method. This allows proper teasing for every conversion and keeps its scope clear.

* Cleaned up BIBO_TYPES
* Added support for more fields
Update rdf gem dependency in Gemfile

There is no reason to limit ourself to this old version.
Add #number
Add #note
Rename #output to #convert!. Move conversion call to #convert!
Fix spelling in comment
Only show load error message when Entry#to_rdf is called
Add #remove_from_fallback and call where appropriate
Refactor Entry#to_xml by introducing Entry::BibTeXMLConverter
Fix #convert!
Add BibTeXML fallback

The fields are added directly to the entry.
BibTeXML would usually require an <bibtex:entry> tag followed by e.g. <bibtex:book>. <bibtex:book> would then contain the fields.
As <bibtex:entry> nor <bibtex:book> are proper predicates, I omitted them for now.
Add support for DC type
Add support DC source

According to http://bibtexml.sourceforge.net/examples.html#dc
Add more BIBO types

Source:
http://biblatex.dominik-wassenhoven.de/download/DTK-2_2008-biblatex-Teil1.pdf
pp. 18-19
Add support for pagination in DC source

Source:
http://biblatex.dominik-wassenhoven.de/download/DTK-2_2008-biblatex-Teil1.pdf
page 17
Add more thesis types
Add DC identifiers
Add bibo publisher
Add DC isPartOf