No getOrder function for btc-markets exchange

[upsatish]

The trader for btc-markets throws the following error

/home/ubuntu/gekko/plugins/trader/portfolioManager.js:355
order => next => this.exchange.getOrder(order, next)
^

TypeError: this.exchange.getOrder is not a function
at next (/home/ubuntu/gekko/plugins/trader/portfolioManager.js:355:36)
at /home/ubuntu/gekko/node_modules/async/dist/async.js:3845:9
at replenish (/home/ubuntu/gekko/node_modules/async/dist/async.js:1030:17)
at /home/ubuntu/gekko/node_modules/async/dist/async.js:1034:9
at eachOfLimit (/home/ubuntu/gekko/node_modules/async/dist/async.js:1061:22)
at /home/ubuntu/gekko/node_modules/async/dist/async.js:1066:16
at _parallel (/home/ubuntu/gekko/node_modules/async/dist/async.js:3844:5)
at Object.series (/home/ubuntu/gekko/node_modules/async/dist/async.js:4663:3)
at Manager.relayOrder (/home/ubuntu/gekko/plugins/trader/portfolioManager.js:358:9)
at Manager.bound [as relayOrder] (/home/ubuntu/gekko/node_modules/lodash/dist/lodash.js:729:21)

[askmike]

In the works, see #936 for details.