Enhance `download()` to `Slicer` class (#11)

* Enhance `download()` to `Slicer` class

* Fix `download()` bug on `Slicer` class

Improve `add()` method of `WaybillCollection` (#10)

Fix errors about `composer apidoc` command (#9)

* Fix apidoc errors

* Add comments of `Waybills` class

* Fix a null point error

Add `Slicer` class and tests (#7)

* Update pdf format(size)

* Isolate `Mpdf` instance

* Add `Slicer` class and tests

* Add `Slicer` usage into `README`

* Remove test files after testing

Add a download function (#6)

Fix doctum errors (#2)