Report correct available balance with node version < 7.

[td202]

Purpose

For node version < 7, the available balance on an account is not returned in the GetAccountInfo query response. This change checks if that is the case and computes the correct balance, instead of simply returning 0.

The main purpose of this is to support the wallet proxy, allowing us to deploy it before node version 7 is deployed.

Changes

• Revise the FromProto implementation for AccountInfo to calculate the available amount if it is not present in the protobuf.

Checklist

• My code follows the style of this project.
• The code compiles without warnings.
• I have performed a self-review of the changes.
• I have documented my code, in particular the intent of the
  hard-to-understand areas.
• (If necessary) I have updated the CHANGELOG.

[drsk0]

Looks good, thanks!