Adding per-request private dictionary

The main goal of this changeset is to add basic facilities to enable the use of a private lexicon that is available only for a single request (it was useful for us in the training phase, so that we were able to provide a private lexicon file for each sentence, solving several ambiguities).
Another goal is to separate lexicon lookup and lts method in the JPhonemiser class.
The changes should not affect other components, as the default behaviour of each class is still the original one.

There is a potential security risk in the strategy adopted for private lexicon: allowing remote users to specify a filename on the server is never a good idea. However I think this can be solved later.