Release v2.4.0

This update adds proxy configuration as a feature of the OpenTok object. (thanks @juandebravo!)

Here is an example of using proxy configuration:

from opentok import OpenTok

opentok = OpenTok(api_key, api_secret)
opentok.proxies = {
  "http": "http://10.10.1.10:3128",
  "https": "http://10.10.1.10:1080",
}
session = opentok.create_session()

The format for the proxy configuration is identical to the format used by the underlying requests library.