Client.py line 1230 -- stream_bars_back

[eyesuk]

Hello Mr Reed
As always thank you very much for your work on these APIs they are nothing short of awesome.
While trying to obtain historical data using stream_bars_back I received a datetime error.
Error:
'datetime.datetime' has no attribute 'datetime'

After some troubleshooting I changed the following lines and it fixed the issue:

Line 16
from datetime import datetime
was changed to:
import datetime

Line 1230
url='stream/barchart/{symbol}/{interval}/{unit}/{bar_back}/{last_date}'
was changed to:
url='stream/barchart/{symbol}/{interval}/{unit}/{bar_back}/{last_date_iso}'

Seems like this fix also fixed others stream_bars_* functions

[akopdev]

@eyesuk here is PR for that #10