Add/closeio

Here's an integration of close.io. 

A few things to note:
- The integration requires the presence of an email for an identify(..), group(..), or track(..) call.
- When identify(..) is called, I first query for the Lead, which is the parent of a Contact, by using the user's email. If that query doesn't return a result, I then query the Close.io Lead endpoint for the company name. If there is a result from the company name query, I create a contact representing that user under that company.
- When group(..) is called, I query for the group using an email address.

Ok, that should do it. Let me know what I can improve on in this integration.
In general, you'll want to avoid large conditionals like this even if you have to repeat a bit of authentication and request creation code. A function ideally does one thing, so that its easily testable. In this case, you have to test 4 different actions based on the request method argument that you pass in.

I recommend you add this code to: 

```js
Close.prototype._createLead = function (payload, settings, fn) {
   this
        .post(path)
        .set(headers(settings))
        // ..
};
```