Add Serialization for DateTime/TimeSpan

[EricBatlle]

First of all: AWESOME editor extension, GREAT f*** job!

Pretty self-descriptive feature, I will love the idea to have something like:

[SerializeField]
private TimeSpan deliveryTimeLimit = new TimeSpan(0, 0, 10);
[SerializeField]
private DateTime startedTime = DateTime.MinValue;

And be able to see it nicely <3