Readme misleading for App Engine requests

[michaelckelly]

From @onejgordon in plaid/plaid-python#76:

On GAE plaid-python uses urlfetch, which returns a _URLFetchResult object. This object has no json() method.

Readme should read (for the GAE case):

client = Client(client_id='***', secret='***', access_token='usertoken')
response = client.connect_get()
if response.status_code == 200:
    transactions = json.loads(response.content)