add support for time in force for Nash,Binance and Coinbase

[jankjr]

This PR adds support for TimeInForce. Different exchanges have different ways of supporting this.

• Nash supports GTC, IOC, FOK and GTT. GTT supports any timestamp as parameter.
• Coinbase supports GTC, IOC, FOK, GTT. GTT only supports three durations: 1 Minute, 1 Hour and 1 day.
• Binance only supports GTC, IOC, FOK.

I feel the most cross exchange way of supporting this would be to represent the GTC, IOC and FOK as normal enums, and GTT as a duration. This would fit the Coinbase model, and for nash the timestamp can be converted by doing a simple current_time + duration calculation.

pub enum TimeInForce {
    GoodTillCancelled,
    ImmediateOrCancelled,
    FillOrKill,
    // Representing 'good till time' as a duration works for both Nash and Coinbase
    GoodTillTime(Duration),
}

This PR also fixes two bugs I found while testing the Coinbase and Binance implementations. Our naivetimestamp deserialization code sometimes fails if the timestamp lands on a whole milisecond. Our Binance client had an endless loop in the get_all_open_orders call.

closes #104