Bugfix for formatting

hi there,

I noticed a problem with the `format()` function ... formatting was only working correctly (with my custom settings) for values below 1000. 

in Holland we use `,` for the decimal separator, and `.` for the [thousands] grouping separator - in the USA (and probably most other places) it's exactly the other way around.

I have a little wrapper module for *currency.js* that changes it's settings to reflect dutch formatting:

```js
var cjs = require('currency.js');

function dutchSettings()
{
	cjs.settings.symbol 	= '€';
	cjs.settings.seperator 	= '.';
	cjs.settings.decimal 	= ',';
}

dutchSettings();

cjs.dutchSettings 	= dutchSettings;
module.exports 		= cjs;
```

an example:

```js
var cjs = require('cjs'); // lets assume that loads the module shown above ^

console.log(cjs(1000).format());
// BEFORE: "1,000.00"
// AFTER: "1.000,00"
```

this PR changes the `format()` function to be agnostic about the `decimal` and `seperator` settings