This makes the following changes:

1.  setVoice takes a Voice object instead of a language string.

    This is to make the API cleaner and allows SpeechSynthesis
    to pass the correct parameter (identifier for SetVoiceByName,
    or name for SetVoiceByProperties) to espeak.

2.  The espeak_SetVoiceByName API is also exposed to support passing
    the voice variant to use.

3.  The age parameter has been re-added with the constants:
    *  SpeechSynthesis.AGE_ANY
    *  SpeechSynthesis.AGE_YOUNG
    *  SpeechSynthesis.AGE_OLD
    based on the behaviour of the eSpeak voice selection algorithm.

NOTE: Due to the way that voice selection is implemented in eSpeak,
if variant is specified, the age and gender cannot be specified and
vice versa.