Do not save processor type, determine it every time when loading.

This allows adding new processor types without changing the format
of the recognizer model. The same strategy is used to load processors
from feature file anyway.
Add experimental support for hidden layer in the network_classifier.

The hidden layer is for experimental use only, because it creates huge
models and increases accuracy only slightly. Nevertheless, it can be
used when best possible accuracy is needed.
Add forgotten $(DIR).
Fix typo Gazeteers -> Gazetteers.
Add documentation about training custom models.