Introduce new "metadata" field to the result data

[perich]

Currently, all of the DEX classes return a result object to every query. It matches the following structure:

result = {
        exchangeName: String,
        totalPrice: Number | null,
        tokenAmount: Number
        tokenSymbol: String,
        avgPrice: Number | null,
        timestamp: Number,
        error: String | null,
      }

This structure doesn't always tell the full story when it comes to price and expectations. For example, the prices reported by the Bancor API are subject to slippage. That slippage isn't reflected at all anywhere in the library. While a user won't experience slippage every time they execute a trade, they should certainly be aware of the possibility. To handle these kinds of non-deterministic details, I am proposing a "metadata" field be added to the result object. This is where we can store useful details about each DEX to help users and developers make more informed decisions when analyzing price data generated from this library.