implemented interface ISender, Sender -> ISender

[unzeitigm]

Hi, in response to #20 and for preserving the BC, I've added an ISender interface and an EmptySender class and I would like to ask if this is a reliable solution?

We've been dealing with this in our company for a while. PPL support told us that we must ommit the Sender when sending shipment package data. When sending pallet data, Sender is required. As for labels, Sender is always required.

How do you feel about this?

[Salamek]

@unzeitigm
Hi, funny thing is that my customers use createPackages with sender filled in since this lib exists and there was no issue reported from PPL 😄

So issue #20 was first time i heard about it...
Having ISender is good thing anyway (when we have this one, maybe we should use Interfaces for all models later...)

If this solution works for you guys, i have only one remark:

• Pls update README.md to reflect this changes (Section Create Packages) to keep our "docs" updated.

Then i will merge this PR

[unzeitigm]

@Salamek
Done. I've also added checking instance type of ISender in API's createPackages() method.

... funny it is indeed :D Firstly they wanted us to remove the Sender ... then another guy didn't care really much, and then the first guy again started complaining and demanded not sending the Sender ... :D