Add ability to use 'to' with mandrill parameters

As this issue say https://github.com/wombat/mandrill_integration/issues/1 sometimes people need to send email also to 'cc' or 'bcc', this pull request add the ability to use the `to` attribute with mandrill parameters, so you can send an email this way:

```
"to": [
  { "email": "spree@example.com" }
  { "email": "alessio.rocco.lt@gmail.com", "type": "cc" }
],
```

or with the string: 

```
"to": "spree@example.com, wombat@example.com",
```