Added short and long tons.

[distantcam]

http://en.wikipedia.org/wiki/Short_ton
http://en.wikipedia.org/wiki/Long_ton

Also known as the US ton and the UK ton, these are currently the bane of my existence.

Thankyou for releasing this library. My team is constantly having to convert things and this will help.

[angularsen]

Thanks for contributing and for following the code style.
Found one mistake though, the short and long tons were implemented the other way around.

You wrote:
Assert.AreEqual(907.18474, oneKg.ShortTons, Delta);
Assert.AreEqual(1016.0469088, oneKg.LongTons, Delta);

One kg is not 907 short tons :-)

Corrected it and merged it.

[distantcam]

Doh! Of course. This is why I need a library to handle this for me. :D