Use Unicode instead of Str in the tutorial examples

Without this change, the "person" and "employee" examples raise an exception when focus is lost in a modified text field:

```
$ python person.py 
Traceback (most recent call last):
  File "/home/warren/local_enaml/lib/python2.7/site-packages/enaml-0.8.0-py2.7-linux-x86_64.egg/enaml/qt/qt_field.py", line 125, in on_submit_text
    self._validate_and_apply()
  File "/home/warren/local_enaml/lib/python2.7/site-packages/enaml-0.8.0-py2.7-linux-x86_64.egg/enaml/qt/qt_field.py", line 95, in _validate_and_apply
    d.text = text
  File "/home/warren/local_enaml/lib/python2.7/site-packages/enaml-0.8.0-py2.7-linux-x86_64.egg/enaml/core/declarative_meta.py", line 67, in declarative_change_handler
    engine.write(owner, change['name'], change)
  File "/home/warren/local_enaml/lib/python2.7/site-packages/enaml-0.8.0-py2.7-linux-x86_64.egg/enaml/core/expression_engine.py", line 210, in write
    pair.writer(owner, name, change)
  File "/home/warren/local_enaml/lib/python2.7/site-packages/enaml-0.8.0-py2.7-linux-x86_64.egg/enaml/core/standard_handlers.py", line 120, in __call__
    call_func(func, (inverter, change['value']), {}, scope)
  File "/home/warren/gitwork/nucleic_enaml/examples/tutorial/person/person_view.enaml", line 17, in <module>
    text := person.first_name
  File "/home/warren/local_enaml/lib/python2.7/site-packages/enaml-0.8.0-py2.7-linux-x86_64.egg/enaml/core/standard_inverter.py", line 47, in load_attr
    setattr(obj, attr, value)
TypeError: The 'first_name' member on the 'Person' object must be of type 'str'. Got object of type 'unicode' instead.
```