Return balance of address via API

[tboeckmann]

Is your feature request related to a problem? Please describe.
In order to show users their current balance in the inbox, we need an endpoint that returns the users current (available to spend) balance in the context of a protocol and network.

Describe the solution you'd like
GET /addresses/{address}/balance?protocol=ethereum&network=ropsten returns:

{
  "balance": 2183846200000000000,
  "unit": "wei"
}

Ethereum
See eth_getBalance here
Returns current balance in wei

Edgeware
E.g. see balance_status here

[sa3664]

@tboeckmann Could you please assign this to me.