Added a central service for doing address balance lookups.
Change the AssetTypesController so that it uses the cetral service for address balance lookups.
Switched LoginController to use the balanceService to look up address balances.
Refit WalletAddressesController to use the balanceService for balance lookups.
Type in error handling.
Added cache of balances.
Updated the cache to cache promise objects, instead of just data.
Added some debug lines for a couple of remaining balance lookups.
Merge pull request #381 from curtislacy/Balance-Cache

Balance cache