Include extra signature info from .wot files.

The 'primary' attribute on edges indicates whether the primary user ID
was signed, and 'level' gives the cert check level, per the .wot
documentation.
Better edge attribute names

gpg refers to the "certification level" as in --ask-cert-level, plus the
code fits on one line if I use cert_level instead of cert_check_level.
Merge pull request #2 from frewsxcv/sigbits

Use extra signature information