Added ability to get short time identifiers for TimeSpan

I needed the ability to get "ms", "s", "h", etc time identifiers.

For example, `TimeSpan.FromHours(2).Humanize()` I'd like to be able to have `"2 days"`or  `"2d"`
Strategy is a OO pattern and that's what we've used it in the codebase for. Calling this enum Strategy is confusing. I would actually like this to not be an enum and change to a class implementing a `ITimeSpanFormatStrategy` interface. That way the customisation of TimeSpan formatting become as simple as implementing a custom strategy. The interface could look something like:

```
public interface ITimeSpanFormatStrategy
{
    string ResourceKeyDifferentiator { get; }
    string Format(TimeUnit timeunit, int unit);
}
```

We need to discuss this a bit more.