get_attribute to consider fillable attributes

[pmn4]

Hey @sdispater, not sure whether you accept outside PRs, but would you consider attributes defined in the __fillable__ array to be honored by the get_attribute method?

class Person(Model):
  __fillable__ = ['name']

person = Person()

print(person.name) # expect None, get AttributeError

---

(I am fairly new to Python and was not able to run the test suite, so while it looks like my tests should work, I wasn't actually able to run them)