refactor all the things

[tacticalchihuahua]

i'm looking at you, @martindale

[tacticalchihuahua]

ping for review by @martindale or @pnagurny

[tacticalchihuahua]

I did this to mimic the more standard approach for node.js libs. You can also do:

var client = new bitpay.Client()

I felt it was better to export an object than just the single constructor, since we will likely add more tools to the exports.

[martindale]

Ah, so an updated pattern (which feels like Bitcore, might be desirable) would be:

var BitPay = require('bitpay').Client;
var bitpay = new BitPay( ... );

This feels reasonable, and might even allow var bitpay = require('bitpay').Server in the future. Neat.