Improve / test mappings from external souces

[vpeil]

check and test the following fix files with more data / bad data:

• fixes/arxiv_mapping.fix
• fixes/bibtex_mapping.fix
• fixes/crossref_mapping.fix
• fixes/epmc_mapping.fix
• fixes/from_datacite.fix
• fixex/wos_mapping.fix

(and stick with one naming schema!)

[vpeil]

There's a new directory xt (= extra tests) for testing importers with the mappings and check for validation. Just run

$ prove -lv xt/

on your machine and see the nice logging information if something fails.

That tool is the basis to improve the fix mappings.