Add accountId to Interests query

The accountId is a required element here, otherwise the endpoint will return: 

```json
{
  "code": 2,
  "message": "Missing required argument: accountId",
  "moreInfo": "http://developer.oanda.com/docs/v1/troubleshooting/#errors"
}
```
Add accountID to price server
Add instrument to URL
Merge pull request #1 from LukasReschke/master

Add accountId to Interests & PriceServer