Added quote endpoint support

[fabianbaier]

This introduces support for the QuoteEndpoint

Example

ac := av.NewClient(APIKey)
quoteValue, err := ac.StockQuote(symbolKey)
		if err != nil {
			log.Debugf("Error StockQuote response: %+v", err)
		}

e.g. for symbolKey = FB a typical Record is: [FB 160.5800 162.4100 160.3100 161.8900 15858468 2019-02-22 160.0400 1.8500 1.1560%]