Add pieces to parcel

[jacobdekeizer]

Description

Add pieces resource to configure the pieces that are going te be shipped.
I also fixed some errors in the readme file, like company_name has to be company, otherwise you get a 400 bad request.

Motivation and context

This change allows a developer to configure the pieces for a parcel shipment.
The package in its current state has a harcoded shipment piece.

How has this been tested?

I created shipments with the new pieces functionality.

Types of changes

What types of changes does your code introduce? Put an x in all the boxes that apply:

• Bug fix (non-breaking change which fixes an issue)
• New feature (non-breaking change which adds functionality)
• Breaking change (fix or feature that would cause existing functionality to change)

Checklist:

• I have read the CONTRIBUTING document.
• My pull request addresses exactly one patch/feature.
• I have created a branch for this patch/feature.
• Each individual commit in the pull request is meaningful.
• I have added tests to cover my changes.
• All new and existing tests passed.
• If my change requires a change to the documentation, I have updated it accordingly.
• My code follows the code style of this project.

[jacobdekeizer]

@mvdnbrk If you create multiple pieces, only 1 trackerCode and labelId is returned from the Shipments class.
How would you like me to solve that to preserve backwards compatibility or can I make a breaking change to return an array of labels.

[mvdnbrk]

Thanks! Could you add tests for this?

[jacobdekeizer]

@mvdnbrk I added the tests.

[mvdnbrk]

Released in v0.5.0.