dns.list is now always an array
Update default hostname to api.dnsimple.com
Fix credentials check
Fix API version in request path
Fix incomplete syntax in requestOK check
Converted module from object to function (#4)

Easy one line setup, multiple instances.

```js
var dns1 = new require('dnsimple')({email: 'foo', token: 'bar'})
var dns2 = new require('dnsimple')({email: 'woof', token: 'bark'})
```
Add support for domain token authentication
Changed order of callback parameters (#3)

`function( err, data ) {}`
Changed all listings from object to array

There are more specific methods when you need to retrieve one item by its ID.
Added .subscription()
Don't continue without credentials