Fix issue #43.

When checking for available balance we should use floor.
Here is an example of the problem: 
:~$ nodejs
amount = 4.55239783;
4.55239783
amount *= 100000000;
455239783.00000006
amount = Math.ceil(amount);
455239784
amount /= 100000000;
4.55239784

As you can see the value is incorrectly rounded and you actually try to sell more than you have on the next step.