Use `invalid?` instead of `!...valid?`
Clean up method_missing

In particular, switch from "method_name" to "attribute" and
"validation_method_name" to "validator".
Implement and use camelize instead of capitalize

This fixes the "date_time" part of #8. Before it was looking for
ActiveInteraction::Date_timeAttr. Now it correctly looks for
ActiveInteraction::DateTimeAttr.