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"`
hmmm, thinking about pluralisation more, if we pluralised m for 2 minutes we'd get 2ms which is then 2 milliseconds. I am not sure how this should work. 

*googling now*