Deprecated automatic conversion from floats for * / % operations

This addresses issue #39, and the other two places that did float conversions where the Decimal class would instead raise TypeError.

Once deprecation period is over, warnings should be changed to a TypeError. However, the best way to do this is probably just to remove the warning, and change `Decimal(str(other))` to `other`, and the underlying Decimal method will raise the appropriate exception (while still allowing things like integers to be used).