README docs for api_call() are incorrect

[andrewkreid]

The README file says

SlackClient.api_call([method, params]) Call the Slack method [method] with the a dict of params in [params]

which implies you'd call it like this

sc.api_call(method="users.list", params=["token":my_token, "user":"U1234567"})

However, you actually call it like this:

sc.api_call(method="users.list", user="U1234567")

[elementc]

#72 fixes this issue.