Oracles

[shargon]

Wait for #219 neo-project/neo#1584

This PR helps with the review of Oracles changes.

You can use this SmartContract for testing

using Neo.SmartContract.Framework;
using Neo.SmartContract.Framework.Services.Neo;

namespace Neo.Compiler.MSIL.TestClasses
{
    public class Contract_Oracle : SmartContract.Framework.SmartContract
    {
        public static object Download(string url, string filterArgs)
        {
            var ret = (object)Oracle.Get(url, null, "filterJson", filterArgs);

            if (ret == null)
            {
                Runtime.Log("Error downloading");
                return null;
            }

            Runtime.Log("Downloaded:" + (string)ret);
            return ret;
        }
        public static object FilterJson(string json, string find)
        {
            var item = (Map<string, int>)Json.Deserialize(json);
            return item[find];
        }
    }
}