#51 better handling of reserved words: runtime support

[Peter A. Bigot]

Can't do anything about Python reserved words, but class members like "content" and "value" that are part of the PyXB reserved word list should use properties, even for function values, to prevent accidents like:

   element.content = my_new_element

when it should have been:

   element.content_ = my_new_element

[Peter A. Bigot]

• status changed from new to accepted
• milestone changed from PyXB 0.7.4 to PyXB 0.7.3

[Peter A. Bigot]

• status changed from accepted to closed
• resolution set to fixed

Done by overloading setattr in _TypeBinding_mixin to raise a BindingError upon any attempt to set a reserved name.