Parity with Network v0.3.0; Bump to v2.1.0

[BHouwens]

Description

This PR brings the ABlock.js SDK up to parity with the changes in Network 0.3.0.

Changelog

• All instances of the terms receipt and Receipt have been changed to item and Item respectively
• Receipt-based terminology has all been renamed to "2 Way" (eg. makeReceiptBasedPayment is now make2WayPayment)
• Metadata is now forced to be null when constructing excess payments for items, as they should never contain metadata
• PubKeyHash has been replaced by Bytes in the P2PKH transaction function

Type of Change

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• Bug fix
• New feature
• Enhancement or optimization
• Documentation update
• Other (please specify)

Checklist

Put an "x" in the boxes that apply. If you're unsure about any of these, don't hesitate to ask. We're here to help!

• I have tested the changes locally and they work as expected.
• I have added necessary documentation or updated existing documentation.
• My code follows the project's coding standards and style guidelines.
• I have added/updated relevant tests to ensure the changes are properly covered.
• I have checked for and resolved any merge conflicts.
• My commits have clear and descriptive messages.