Update doc examples for correct imports

[myagley]

Update the examples to import all of the required classes. For example, the code below doesn't import DataPoint

from datetime import datetime
from tempodb import Client

client = Client("api-key", "api-secret")

data = [
    DataPoint(datetime(2012, 1, 1, 1, 0, 0), 12.34),
    DataPoint(datetime(2012, 1, 1, 1, 1, 0), 1.874),
    DataPoint(datetime(2012, 1, 1, 1, 2, 0), 21.52),
]

client.write_key("my-custom-key", data)