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-calculate-price.js

@@ -0,0 +1,78 @@
+import test from 'ava'
+import rewire from 'rewire'
+
+function rewireCalculatePrice (commonMock) {
+  const itbit = rewire('../../lib/plugins/exchange/itbit/itbit')
+
+  itbit.__set__('common', commonMock)
+
+  const calculatePrice = itbit.__get__('calculatePrice')
+
+  return calculatePrice
+}
+
+test('calculate minimum available price for buy', async t => {
+  const commonMock = {
+    request () {
+      return Promise.resolve({
+        asks: [
+          [2, 10],
+          [4, 15],
+          [4.5, 17]
+        ],
+        bids: []
+      })
+    }
+  }
+
+  const calculatePrice = rewireCalculatePrice(commonMock)
+
+  let price = await calculatePrice('buy', 'XBTUSD', 20)
+
+  t.is(price, 4)
+})
+
+test('calculate minimum available price for sell', async t => {
+  const commonMock = {
+    request () {
+      return Promise.resolve({
+        bids: [
+          [2, 10],
+          [3, 15],
+          [4.5, 17]
+        ],
+        asks: []
+      })
+    }
+  }
+
+  const calculatePrice = rewireCalculatePrice(commonMock)
+
+  let price = await calculatePrice('sell', 'XBTUSD', 20)
+
+  t.is(price, 3)
+})
+
+test('throw error on insufficient trade depth', async t => {
+  t.plan(1)
+
+  const commonMock = {
+    request () {
+      return Promise.resolve({
+        asks: [
+          [2, 10],
+          [4, 15],
+          [4.5, 17]
+        ],
+        bids: []
+      })
+    }
+  }
+
+  const calculatePrice = rewireCalculatePrice(commonMock)
+
+  calculatePrice('buy', 'XBTUSD', 100)
+    .catch(err => {
+      t.is(err.message, 'Insufficient market depth')
+    })
+})