Fix #173: HTTPS request doesn't work when disable_ssl_certificate_validation=True

Sets `SSLContext.verify_mode` to `ssl.CERT_OPTIONAL` if `disable_ssl_certificate_validation` is `True` otherwise it is set to `ssl.CERT_REQUIRED`.
```python
Traceback (most recent call last):
  File "<string>", line 1, in <module>
  File "/Users/carlos/Public/zulu/oss/httplib2/python3/httplib2/__init__.py", line 851, in __init__
    check_hostname=True)
  File "/usr/local/Cellar/python3/3.4.2_1/Frameworks/Python.framework/Versions/3.4/lib/python3.4/http/client.py", line 1211, in __init__
    raise ValueError("check_hostname needs a SSL context with "
ValueError: check_hostname needs a SSL context with either CERT_OPTIONAL or CERT_REQUIRED
```