fx: add CoinGecko and CoinCap

somewhat based on
Electron-Cash/Electron-Cash@b3c76cd311
Electron-Cash/Electron-Cash@adf8f943a1
Electron-Cash/Electron-Cash@8879e41fa0

electrum/currencies.json

@@ -386,6 +386,9 @@
         "TWD",
         "USD"
     ],
+    "CoinCap": [
+        "USD"
+    ],
     "CoinDesk": [
         "AED",
         "AFN",
@@ -555,6 +558,62 @@
         "ZMW",
         "ZWL"
     ],
+    "CoinGecko": [
+        "AED",
+        "ARS",
+        "AUD",
+        "BCH",
+        "BDT",
+        "BHD",
+        "BMD",
+        "BNB",
+        "BRL",
+        "BTC",
+        "CAD",
+        "CHF",
+        "CLP",
+        "CNY",
+        "CZK",
+        "DKK",
+        "EOS",
+        "ETH",
+        "EUR",
+        "GBP",
+        "HKD",
+        "HUF",
+        "IDR",
+        "ILS",
+        "INR",
+        "JPY",
+        "KRW",
+        "KWD",
+        "LKR",
+        "LTC",
+        "MMK",
+        "MXN",
+        "MYR",
+        "NOK",
+        "NZD",
+        "PHP",
+        "PKR",
+        "PLN",
+        "RUB",
+        "SAR",
+        "SEK",
+        "SGD",
+        "THB",
+        "TRY",
+        "TWD",
+        "USD",
+        "VEF",
+        "VND",
+        "XAG",
+        "XAU",
+        "XDR",
+        "XLM",
+        "XRP",
+        "ZAR"
+    ],
     "Coinbase": [
         "AED",
         "AFN",

electrum/exchange_rate.py

@@ -248,6 +248,24 @@ class Coinbase(ExchangeBase):
         return {ccy: Decimal(rate) for (ccy, rate) in json["data"]["rates"].items()}
 
 
+class CoinCap(ExchangeBase):
+
+    async def get_rates(self, ccy):
+        json = await self.get_json('api.coincap.io', '/v2/rates/bitcoin/')
+        return {'USD': Decimal(json['data']['rateUsd'])}
+
+    def history_ccys(self):
+        return ['USD']
+
+    async def request_history(self, ccy):
+        # Currently 2000 days is the maximum in 1 API call
+        # (and history starts on 2017-03-23)
+        history = await self.get_json('api.coincap.io',
+                                      '/v2/assets/bitcoin/history?interval=d1&limit=2000')
+        return dict([(datetime.utcfromtimestamp(h['time']/1000).strftime('%Y-%m-%d'), h['priceUsd'])
+                     for h in history['data']])
+
+
 class CoinDesk(ExchangeBase):
 
     async def get_currencies(self):
@@ -277,6 +295,25 @@ class CoinDesk(ExchangeBase):
         return json['bpi']
 
 
+class CoinGecko(ExchangeBase):
+
+    async def get_rates(self, ccy):
+        json = await self.get_json('api.coingecko.com', '/api/v3/exchange_rates')
+        return dict([(ccy.upper(), Decimal(d['value']))
+                     for ccy, d in json['rates'].items()])
+
+    def history_ccys(self):
+        # CoinGecko seems to have historical data for all ccys it supports
+        return CURRENCIES[self.name()]
+
+    async def request_history(self, ccy):
+        history = await self.get_json('api.coingecko.com',
+                                      '/api/v3/coins/bitcoin/market_chart?vs_currency=%s&days=max' % ccy)
+
+        return dict([(datetime.utcfromtimestamp(h[0]/1000).strftime('%Y-%m-%d'), h[1])
+                     for h in history['prices']])
+
+
 class itBit(ExchangeBase):
 
     async def get_rates(self, ccy):