Proposal: add the currency to the "income" field

this could be helpful for robots and websites handling the format to know what currency the JSONHire is talking about.

It could be a simple string field, with the currency I[SO 4217 Alpha Code](http://www.thefinancials.com/?SubSectionID=curformat) (in uppercase), such as:

```
currency: "USD"
```

for instance:

```
"income": {
"from": 100000,
"to": 120000,
"currency": "EUR"
}
```