Add EC support to PKey

[mrjefftang]

Depends on pyca/cryptography#933

[exarkun]

Please add unit tests and documentation: https://github.com/pyca/pyopenssl/blob/master/CONTRIBUTING.rst

[mrjefftang]

Made the changes you asked for.

I'm not quite sure if it makes sense adding another parameter, 'curve', to generate_key or to reuse the 'bits' parameter.

I stuck with adding another parameter with a default of None since the parameter types are different.

[exarkun]

Why is this closed?

[mrjefftang]

I had to clear out my repository so the branch it referred to no longer existed.

Also I'm thinking there's probably a better way of going about this such as creating a subclass of PKey like ECPKey which just overrides the necessary methods.