Allow creating a Context with SSLv2_METHOD

Hi there,

It appears that it's not possible to create a new Context with SSLv2_METHOD.  I realize this is horribly insecure, but I'm attempting to use pyOpenSSL to scan for insecure HTTPS settings, and this error occurs.  Here's the message:

    $ python
    Python 2.7.6 (default, Feb 19 2014, 19:58:32)
    [GCC 4.2.1 Compatible Apple LLVM 5.0 (clang-500.2.79)] on darwin
    Type "help", "copyright", "credits" or "license" for more information.
    >>> from OpenSSL import SSL
    >>> ctx = SSL.Context(SSL.SSLv2_METHOD)
    Traceback (most recent call last):
      File "<stdin>", line 1, in <module>
      File "/Users/andrew/.virtualenvs/ssl/lib/python2.7/site-packages/OpenSSL/SSL.py",
     line 261, in __init__
        raise ValueError("No such protocol")
    ValueError: No such protocol
    >>>

I've attached a patch that fixes this.

Thanks! :smile: