Add missing signing algorithms

Fixes #31 (closed).

This adds the remaining signing algorithms and ajust the test to error if gnutls lists an algorithm that is not in the bindings (similar to the pk-algorithms one).

The list now matches gnutls 3.8.9. I took the liberty to reorder the list to match the enum declaration in gnutls.h and added the same empty lines to help organize the list.

The enum list now contains 48 + 1 entries (for unknown). The enum goes from 0-49 but number 19 is missing. The (sign-algorithm-list) contains 48 elements.

There are probably some issues with some of the algorithms only being available in very recent gnutls versions.