Add Electrumx Get Merkle Branch to bch-js

[christroutner]

This task depends on this previous task being completed first.

The scope of this task is to add an Electrumx.merkleBranch() method to the Electrumx class in bch-js.

You should submit a PR to edit the electrumx.js file in bch-js to add this new merkleBranch method. Follow the same patterns in that file that is used for the utxo endpoint: It should accept a single txid or an array of txids.

Acceptance Criteria

• Add an merkleBranch method to the Electrumx class in the electrumx.js file.
• Add api-doc documentation and examples to the method in that file as well.
• Add integration tests for both mainnet and testnet
• Add unit tests with mocks.

You should be able to duplicate the patterns for the existing endpoints.