Add getCode() method to JsonRpcProvider

[arimgibson]

Should return the contract code of a given address on a given block. Uses the eth_getCode RPC call.

ethers.js equivalent - provider.getCode

const essentialEth = require('essential-eth');
const provider = new essentialEth.JsonRpcProvider();
provider.getCode('0x6fc21092da55b392b045ed78f4732bff3c580e2c', 14642060);
// '0x6060604052...'
// Value extremely long and spans multiple lines, this is the first bit of data returned

[dawsbot]

@arimgibson Keep in mind that essential-eth does not yet support ENS until #23 is completed. So the code example above would need the full "0x" address instead of the ".eth" domain