Simplify the validator API and make it compatible with FormEncode.

It is now always sufficient to call to_python() in order to both convert
and validate a value. For subclassing validators, we use the new API of
FormEncode 1.3, i.e. _validate_python() instead of validate_python().
The old method method name is still supported, but prints a warning.
Merge pull request #75 from Cito/develop

Simplify the validator API and make it compatible with FormEncode (#65).