Support for traditional OpenSSL and PKCS8 RSA private key serialization

This temporarily uses the backend to do it until the alternate approach is ready. DSA/ECDSA and public key support will follow if this is deemed worthy.
I'm unsure about what to do here. We've talked about exposing the entire complexity of this in the past but we also want to be capable of interoperability without having a PhD in serialization techniques. Maybe `enctype` could, in addition to the "simple" types, take an option like:

```python
class FancySerialization(object):
    def __init__(self, cipher_algorithm, bit_length, mode, hash_algorithm, pbe, password):
         # some stuff
```

Can't say I like this idea, so maybe others have a better one?