Include quantity.rb

Would be great to also include a sample file containing all the examples

I am not too sure it would be a good thing.

Implementing class doing nothing except what described in the convention (e.g. construction, equality and so on) will demonstrate some not-that-useful code, and implementing full-force Quantity with units, conversions, and stuff will expose too huge area to review a discuss besides points illustrated. So I believe short and focused inline examples is the best we can go with.

At least, I want covnention to be discussed and clarified first.