Added the appropriate npm dependencies

Don't know if this is the best way of handling these dependencies (perhaps they go in package.json), but they were required for stellar-lib to install correctly.
The dependencies should be installed when you run ```npm install```, because they are listed as "devDependencies" in [```package.json```](https://github.com/stellar/stellar-lib/blob/master/package.json#L27).

The other changes look good though. Thanks!