expose a dictionary API to the tts engines

[rhdunn]

The engines API should have dictionary support that allows:

• adding/updating a word to the dictionary;
• reloading the dictionary set;
• looking up the pronunciation of a given word.

This allows support for user and document-based dictionaries, as well as supporting pronunciation verification tools and building exception dictionaries from the results of the verification tool.

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[rhdunn]

Depends on #38 (Phoneme Model)