Add DOIs and eprint links to non-articles.
Relevant to issue #6, but does not, I think, close it.

We include a format.doi, format.eprint and format.url in all entry types
_except_ {article}.  These will be redundant in most cases: at present (2013)
the majority of things that have DOIs, or which are submitted to arXiv, are
articles; however it's not impossible for other things to have the extra links,
so we keep the printed links for the sake of consistency.

In articles, any DOI is linked to the journal name, and any 'adsurl' field
(which will generally be present on BibTeX entries downloaded from ADS) will be
linked to the journal and page number (suggestion from Keith).  Note that we do
this _only_ if there is an 'adsurl' field present; it's tempting, but surely too
errorprone, to start synthesising ADS URLs from scratch.
Handle presence/absence of eprint prefix.

Addresses issue #7, but I'd like some discussion before closing it.

This makes minimal changes to the .bst file, but replaces \eprint by a
two-argument version, and \doi by a version with an optional first argument.
These two macros are removed from the .bbl output, and replaced with same-name
ones in the preamble of mn2e-example.tex; these should be considered for
migration to mn2e.cls.

These appear to work, but they do mean that the mn2e.bst output would be usable
_only_ with mn2e.cls.

I've implemented support for archivePrefix="dblp", not because I think very many
people will want to use a CS bibliography archive, but to illustrate how support
for other archives is provided.  Ie, it's not 'arXiv or bust'.