Fixes #1533 -- Initial work at parsing ECDSA public keys in OpenSSH format

I've seen no evidence that rsa1 keys are of any notable popularity --
neither Go nor paramiko nor conch support them as far as I ca tell.

On Wed Dec 24 2014 at 3:44:17 PM Mohammed Attia <notifications@github.com>
wrote:

> In docs/hazmat/primitives/asymmetric/serialization.rst
> <https://github.com/pyca/cryptography/pull/1576#discussion-diff-22265006>:
>
> > @@ -201,9 +201,6 @@ OpenSSH Public Key
> >
> >  The format used by OpenSSH to store public keys, as specified in :rfc:`4253`.
> >
> > -Currently, only RSA and DSA public keys are supported. Any other type of key
> > -will result in an exception being thrown.
> > -
>
> A look at man ssh-keygen specifies dsa, ecdsa, ed25519, rsa and rsa1,
> rsa1 refers to the format of the first version of the OpenSSH protocol
> (repeated here for emphasis and for others).
>
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> <https://github.com/pyca/cryptography/pull/1576/files#r22265006>.
>