Beginning of the buying API, look at following comments
In case of the error "cannot find symbol", do a "play clean"
For now it doesn't do anything, it only shows a message with the price and the
number. Next step is to link it with the user stored in the session and do every
operations needed. The api can take an optionnal parameter (?number=...) to give
the number of stocks to buy. It asks Yahoo API for the price, to avoid giving it
in the request and cheating.