Upload feature

Implemented basic feature to upload files.

Use:

\Httpful\Request::post($this->url)
                    ->addHeader('Accept', 'application/json')
                    ->body(array())
                    ->attach(array('indexToPost' => '/path/to/file'))
                    ->send()

The content type should be multipart/form-data
Method should be POST
Data should be array