Add balance transfer to settings

Add a "Transfer Balance" setting that calls the account merge API to transfer the user's balance to a different account. The API call removes the account entry from the ledger, so when this request is complete all the wallet's gateways are removed to reflect the fact that the trust lines no longer exist.

Needs UX feedback. I used a trashcan icon, but that's probably not the correct icon until we make this a feature that deletes the user account also.

Fixes #10.
Use ```StellarNetwork.sendTransaction()``` to avoid the event promise boilerplate above.