Changed a few fields to be optional.

Found a fairly difficult bug involved with Field instances
being shared across Records. The issue is that Field instances
are static. I either need to implement a way to instantiate
copies of all the Fields per-record, or write a wrapping
interface which provides a unique value store on a per-Record
basis.

setup.py

@@ -1,6 +1,6 @@
 from distutils.core import setup
 setup(name='pyaccuwage',
-    version='0.0.3',
+    version='0.0.4',
     packages=['pyaccuwage'],
     zip_safe=True,
     )