X509Backend support in OpenSSL backend

This PR adds a dependency on `enum34`. I am expecting some improvements to be needed in the docs along with potentially some comments required to explain the purpose of some of the tests.

TODO:
- [x] Resolve issue with EC public keys where curve name is not set. (resolved by raising error for now)

Depends on #1498.
The second sentence doesn't read super naturally, should probably say "See the X.509 documentation for more information" with "X.509 documentation" linkified.