Support new DBLP publication types.

[Traderjoe95]

The current version of DBLP contains some new publication types, which are not supported in the parser. Specifically, the unsupported top-level xml element types are

• phdthesis
• mastersthesis
• person
• data

For reference, see the DTD at http://dblp.uni-trier.de/xml/dblp.dtd

[kenkangxgwe]

see #3