Add support for named parameters when using the JSON-RPC interface

[ruimarinho]

Continuation of #33 with improved tests, README updates and version.exec fixes.

This PR adds support for JSON RPC named parameters (only available in Bitcoin Core >=0.14.x).

const balance = await new Client(config.bitcoind).getBalance({
  account: '*',
  minconf: 0
});

// Same as:
const balance = await new Client(config.bitcoind).getBalance('*', 0);

[joaopaulofonseca]

WDYT of adding an extra assert to test that, for legacy reasons:
client.getBalance({ account: '*', minconf: 0 }) equals client.getBalance('*', 0);