Initial implementation of the uploadFile method for Leads

Hi again,

I had a task related to uploading a file to Zoho, so decided to implement it. The new method has 4 input arguments. The argument `descriptor` supports properties below (see https://github.com/felixge/node-form-data#alternative-submission-methods):

```js
{
    filename: 'unicycle.jpg',
    contentType: 'image/jpg',
    knownLength: 19806
  }
```

No specs yet, please review if it makes sense.

---
Thanks,
Maksim