Add standardized toXec and toSatoshis functions. Cashtab has iterated through too many of these functions. They should be simple, tested, and live in one place.

For now -- just use the BigNumber library, as this can't be deprecated for some time.

This diff does not implement the functions. This is part of a series of diffs working to standardize balance headers in Cashtab and remove functions from utils.

npm test