avail variable is no longer part of OandaSizer

[ebredy]

Part of this code logic that initialized avail was removed and is throwing an error:

btoandav20/btoandav20/sizers/oandav20sizer.py

Line 49 in 5a57dfa

size = avail * (self.p.percents / 100)

I looked through the history, and I'm not to clear as to the reason why it was removed and the logic that should be there to properly initialize avail variable