Provide open_positions as a standardized method

[metaperl]

I am converting a script written for Bittrex to run on BitEx.

Bittrex has an API method cancelAllOrders which basically gets all open transactions and then cancels each one.

There is a standardized method for cancelling an order cancel_order ... however I do not see a standardized method to list open positions.

This issue is somewhat related to Promote closed_positions() to standardized method

[deepbrook]

Hey @metaperl ,
This is already implemented on 2.0.0/dev and will be part of said release.

Cheers,
Nils