Add OHLC retrieval func (GetHistoricCandles) to all exchanges and expose it as a wrapper func (#414)

* initial wiring to providegethistoricalcandles

* initial wiring to providegethistoricalcandles

* initial wiring to providegethistoricalcandles

* gethistriccandles work from cli using hard coded inputs

* gethistoriccandles RPC service and CLI working fine for coinbasepro

* fixed unit test

* input check on grpc for gethistoriccandles

* updated deps

* fixed the return value when a method is not yet implemented

* code review: fixed CLI input check and int32->int64

* code review: handling wrong exchange name

* added check on granularity and allowing start and end being empty

* code review: removed currency2

* code review: dependency reverted

* improved func comment

* typo in func comment

* get historic values tests

* unit tests for get historical rates on coinbasepro

* using time format time.RFC3339

* names to camel case and improved comments

* test cleanup

* changed to camel case

* added InArray tests

* dropped not needed string time

* enforced use of int64

* fixed make check

* cleaned up code organisation to be consistent

* fixed Travis remarks

* more Travis remarks

* added comments

* regenerated proto files after merge

* linter fix

exchanges/bitstamp/bitstamp.go

@@ -64,6 +64,11 @@ type Bitstamp struct {
 	WebsocketConn *wshandler.WebsocketConnection
 }
 
+// GetHistoriCandles returns _rangesize_ number of candles for the given _granularity_ and _pair_ starting from the latest available
+func (b *Bitstamp) GetHistoricCandles(pair currency.Pair, rangesize, granularity int64) ([]exchange.Candle, error) {
+	return nil, common.ErrNotYetImplemented
+}
+
 // GetFee returns an estimate of fee based on type of transaction
 func (b *Bitstamp) GetFee(feeBuilder *exchange.FeeBuilder) (float64, error) {
 	var fee float64