Switch from `input_filters` to `input_filter_specs`

We discovered on Friday that ZF 2.3.0 now has an InputFilterManager, registered as a service provider... using the configuration key `input_filters`. This conflicts with the zf-content-validation module, but since ZF2 is upstream, the change needs to occur here.

This PR does two things:

- The `InputFilterAbstractServiceFactory` now checks for the `input_filter_specs` key instead of `input_filters`.
- The `InputFilterAbstractServiceFactory` is now registered as an abstract factory for the `InputFilterManager`, instead of the `ServiceManager`.
- The `ContentValidationListenerFactory` now injects the `InputFilterManager` instead of the `ServiceManager` into the `ContentValidationListener`.

These changes should keep the module compatible and consistent with ZF2.