Use extra signature information

The signatures file includes a few extra bits of information for each signature that we could include in the graph data, specifically:

```
  The 4-bit signature type is interpreted as:

    If the signing key has signed the primary user ID and one or more
    other IDs, all signatures except the one on the primary user ID
    are ignored.

    If and only if the signing key has signed the primary user ID, bit
    2 is set and the two least significant bits are set to the cert
    check level (0-3) of that signature.

    If the primary user ID is not signed, one or more other user IDs
    are. Bit 2 is not set, but the two least significant bits are set
    to the highest cert check level of those signatures.

    Bit 3 is reserved and might be given a meaning in future 0.2.x
    versions. Set to zero when writing and ignore when reading.
```