feed: make price typecheck less strict

The price conversion is now able to deal with subclasses of int, float,
str and not only  the exact types.
feed: reject bool values for prices

fixes the test broken by previous commit
add price conversion tests for int, str, float subtypes
feed: less strict typechecks for price dict building

In a similar fashion as already implemented for convertPrice,
buildPrices now accepts subtypes of str, int, float -- and dict
Merge pull request #3 (feed: make price typecheck less strict)

The price conversion is now able to deal with subclasses of int, float,
str and not only the exact types.

This gives greater flexibility when working with subclasses, as you do not
have to explicitly convert to the corresponding supertype.