Add PoW for browser parallelized with Web Workers 

This method should take two params:
- `difficulty` (number of zeros in hash)
- and `buffer` of data.

The data will be concatenated with a nonce and hash will be calculated. The hash should contain the `difficulty` number of `0` in the beginning.

Use https://www.npmjs.com/package/worker-task-runner for parallelizing the task.