add support for tagging

[haraldschilly]

the publications are currently not tagged. categorizing them by tags would make it possible to list only an interesting subset. the underlying python module pybtex supports arbitrary fields, hence this would work:

@article{abc,
     ...
     tags = { tag a, nt:something, linear}
}

then, extract they keywords like this: map(lambda t : t.strip(), entry.fields["tags"].split(",")) and build a suitable data structure.