To make debugging of the heartbeats a bit easier, this commit adds a stringify package which can show even very complex objects in a nice readable format like:

Heartbeat {
    nodeId: "c0a2918db00514d655f2b82199c6aedda234f807"
    mainStatement: OpinionStatement {
        previousStatementHash: <nil>
        nodeId: "c0a2918db00514d655f2b82199c6aedda234f807"
        time: 1567213114
        toggledTransactions: [
            ToggledTransaction {
                transactionId: 0x29f54622b0df47c864ceca1a5bb4ed84fb7dbbc1b4a6d9a029f79572b86f4d3c68bcca30b08953ac9d217a667facac7d4aa0
                initialStatement: true
                finalStatement: false
            },
            ToggledTransaction {
                transactionId: 0x774f74040c249b73ec4c390352e3191438899c166863b86765e1ef8546ea735bd0ff28dea6e35c9b7d66b35a5703b0c7fe63
                initialStatement: true
                finalStatement: false
            },
        ]
        signature: 0x57fa4ba799a2312b4e8561b144d9386703fce7f128c30c9ec2eeec73693878704d0a176cd4563f5afa8f5ceabdd2552f42bd17cd82363fc0bd1c1386edf914d401
    }
    neighborStatements: map{}
    signature: <nil>
}