Support itbit exchange/ticker. Additions (#232)

* Initial itBit implementation

* Add tests for calculatePrice in exchange/itbit.js

* Add etherium to supported crypto codes in common/itbit.js

* Apply standardjs to itbit files

* itbit trade test. not important

* test getCurrencyRates in ticker/itbit.js

test/unit/itbit-trade.js

@@ -0,0 +1,35 @@
+import test from 'ava'
+import rewire from 'rewire'
+import BN from '../../lib/bn'
+
+function rewireTrade (commonMock, calculatePrice = () => Promise.resolve(15)) {
+  const itbit = rewire('../../lib/plugins/exchange/itbit/itbit')
+
+  itbit.__set__('common', commonMock)
+  itbit.__set__('calculatePrice', calculatePrice)
+
+  const trade = itbit.__get__('trade')
+
+  return trade
+}
+
+test('should handle itbit error response', async t => {
+  t.plan(1)
+
+  const commonMock = {
+    mock: true,
+    authRequest () {
+      return Promise.reject(new Error('The wallet provided does not have the funds required to place the order.'))
+    },
+    buildMarket () {
+      return 'XBTUSD'
+    }
+  }
+
+  const trade = rewireTrade(commonMock)
+
+  trade('buy', { walletId: 'id' }, BN('93410'), 'USD', 'BTC')
+    .catch(err => {
+      t.regex(err.message, /wallet provided/g)
+    })
+})