Pdf add DOI check & search

I modified your xmp parser a little bit to check DOI and added a parser in usual info in pdf if DOI is not found in metadata.
The rationale is that it is better to check this in pdf as the pdf could not have an xmp_metadata value.
Only DOI will be checked as other identifiers are not really interesting. They could be added later if there is a need.

By the way, I have a strange behavior of the metadata parser which is adding Unknown if I read metadata from pdf to the old title like 'Old title - Unknown' when no title is found. Is this normal?