Add AccountTransactions

[micvbang]

Hello,

I added the AccountTransactions endpoint.

It is somewhat convoluted compared to the rest of the endpoints. The reason is explained in my comment over the accountTransactionResult type.

An example of inconsistent data returned by Bitstamp:

{
	"fee": "0.00000000",
	"order_id": 294995485,
	"eth_btc": 0.06907000,
	"datetime": "2017-09-15 15:52:47",
	"usd": 0.0,
	"btc": "0.01602514",
	"eth": "-0.23201300",
	"type": "2",
	"id": 21435498,
	"eur": 0.0  // <---
}, {
	"fee": "0.00",
	"order_id": 294984225,
	"datetime": "2017-09-15 15:46:53",
	"usd": 0.0,
	"btc": "0.03200000",
	"btc_eur": 3125.00000000,
	"type": "2",
	"id": 21433500,
	"eur": "-100.00"  // <---
}, {
	"fee": "0.00",
	"order_id": 294977112,
	"datetime": "2017-09-15 15:43:24",
	"usd": 0.0,
	"btc": 0.0,
	"eth": "0.46403712",
	"eth_eur": 215.30000000,
	"type": "2",
	"id": 21432376,
	"eur": "-99.91"  // <---
},

Here, we see that the "eur" property is returned both as a number and as a string.

[micvbang]

This PR was moved from master to a separate branch, and moved to #14.