Many updates based on @seancribbs's response
- No longer preconfigure `storage_backend` and don't distable HTTP listener
- Renamed `security_creds` to `credentials` everywhere
- Moved `security_auth_headers` to transport
- Make PyOpenSSL required in `setup.py`
- Check SSL version on import instead of upon initialization
- Added `SecurityError` subclass of `RiakError`
- Removed security credentials from the transport classes
- Simplified some error reporting in PBC connection
- Cleaned up PEP8 spacing
- Reorged some of the PBC connection messages