Allow get_product_trades() to be paginated

[tamersaadeh]

This implements the pagination of trades, as per the api docs (https://docs.gdax.com/#get-trades).

[danpaquin]

This is needed, but I have a couple concerns:

1. r.raise_for_status() is really bad at handling issues within a wrapper. I would rather have error handling similar to the one implemented in #260 or not at all.

2. rate-limiting should be handled outside of each method call. This is a bigger project so we should not worry about it here, but please do not use time.sleep() within any method.

[tamersaadeh]

r.raise_for_status() is really bad at handling issues within a wrapper. I would rather have error handling similar to the one implemented in #260 or not at all.

I only used it for debugging, and I've removed it. (I used the authenticated_client as a template)

rate-limiting should be handled outside of each method call. This is a bigger project so we should not worry about it here, but please do not use time.sleep() within any method.

But since this will perform recursion it will always be rate limited. I have commented it out but I think we need a way to handle it properly.

PS if #260 gets merged I could adapt the code to use it