Convert all free functions to methods on API context object
Fixes #2 Add HTTP CONNECT proxy support to duo_client_python

Summary:
httplib exposed built-in CONNECT support in Python 2.7. An equivalent
private API is available in 2.6.3+. CertValidatingHTTPSConnection
overrides the standard HTTPConnection and breaks CONNECT tunneling, so
this adds the necessary integration there, too.

Test Plan:
Manually submit requests with ca_certs=filename,
ca_certs='DISABLE', and ca_certs='HTTP' through proxy.py from
https://github.com/senko/tornado-proxy.git