Add fieldName as first parameter to Bind and Binder.

Henceforth, types can be re-used without the need to modify them every time
their field name changes. Enables types to be used more than once in a single
form.

closes #3
Improve comments.
Merge pull request #15 from bhcleek/binder-with-fieldname

Add fieldName as first parameter to Bind and Binder.