get_top_markets api

[oxarbitrage]

a top_markets api is suggested by @svk31 here #171 , i think is a good idea but at that time there was no acceptable way to do it. now @abitmore is after improving the performance of get_ticker here #509 and after it is done we will be in good position to make this top markets by volume call.

i think the toplist should be made against CORE(BTS) in order to make sense but the details are open for discussion.

this ticket is open to track the progress of such a new call only after #509 is done work on this can be started.

[oxarbitrage]

closed by #737