Add eid-mw: authenticate & sign with Belgian eID identity cards

Adds limited but useful support for Belgian electronic ID cards.

Limited for now, because:
- While a Firefox extension is included and installed, no effort is made to integrate it. I use Chromium.
- The library should work with all NSS-compatible browsers, but must be registered manually by and for each user. Instructions are included.
Sounds like a reasonable solution.

I'll let you know when I either get it to work, or fail miserably. Either should be educational. Thanks for the help so far!