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:
- [ ] Resolve issue with EC public keys where curve name is not set.

Depends on #1498.
@public I just tried this with an EC cert and couldn't replicate. Do you have one that exhibits this problem I could use for testing?