Slopes is now Avalanche.js

This is a full rename of "Slopes" to the new and wonderful name, "Avalanche.js"

There's some changes that come with this.

• Now you install via npm install --save avalanche
• You import the "avalanche" package in your code instead of "slopes"
• slopes.Slopes is now avalanche.Avalanche
• A few functions in the AVM module have been renamed in both the AVMAPI and the UTXOSet classes:

makeBaseTx -> buildBaseTx
makeCreateAssetTx -> buildCreateAssetTx
makeNFTTransferTx -> buildNFTTransferTx

Most things should remain the same.

• trying 2.0.0 so to not conflict with slopes 7af9b9c
• updating version to prevent collision 8b64ee9

v1.0.0...v2.0.0