Issue/#34 - Passing a list of callables for validation

Added the possiblity to pass a list of callable for validation as asked here: https://github.com/sloria/marshmallow/issues/34
Use `isinstance` for type-checking here. It should be possible to pass a tuple of functions. You can therefore use `isinstance(self.validate, (list, tuple))` or even `utils.is_iterable_but_not_string(self.validate)` to be even more robust (that would handle generators, for example).

Also, the list comprehension is unnecessary here. `[i for i in self.validate]` is the same as `self.validate` in this context.