Consider Adding QuestTrade as alternative to TradeKing for Canadians [feature]

[richjdsmith]

Their API docs are here: http://www.questrade.com/api/documentation/getting-started

I am very new to coding but will give it a shot. May need someone with better code eyes to review.

[interwho]

@richjdsmith Want to work together on this?

[interwho]

I forked this and added support for Questrade, however, I had to rip out support for TradeKing as a result since things were too tightly integrated, and Questrade's API is missing a bunch of features that TradeKing's has.

See: https://github.com/interwho/trump2cash

I suppose trading.py can be modularized further to support both, with the user being able to choose which to use.

[maxbbraun]

Consolidating into #22.