Account transactions

[micvbang]

Hello,

I added support for the AccountTransactions endpoint.

On the surface, my implementation looks somewhat convoluted compared to the implementation of the rest of the endpoints. The reason is explained in my comment above the accountTransactionResult type:

// accountTransactionResult is used internally to ease the process of unmarshalling.
// For some reason, for the transaction endpoint, Bitstamp is sends numbers
// encoded as both strings and floats, intermingled. This means that we cannot
// use the `json:"name,string"` automagic conversion here.
type accountTransactionResult struct {
    ...
}

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.