Use deterministic signatures for ECDSA keys.

This is now considered the preffered approach across the board for ECDSA. This
is because with the traditional, random "k" parameter for ECDSA, any entropy
problems at all, even a single bit, about "k", results in a complete compromise
(see https://en.wikipedia.org/wiki/ECDSA#Security). The deterministic algorithm
doesn't have this downside.
Merge branch 'master' into ecdsa-deterministic

Conflicts:
	paramiko/ecdsakey.py
Merge branch 'master' into ecdsa-deterministic

Conflicts:
	paramiko/ecdsakey.py
Changelog, closes #299