add features and fix implementations.

* update document for installation.
* modify function `speech-synth' to possess multiple *Speech Synth* buffers.
* change implimentations of `speech-synth-from-buffer',
  `speech-synth-from-region', `speech-synth-execute-synthesis' and
  `speech-synth-get-string'.
* remove functions of `speech-synth-japanese-from-buffer',
  `speech-synth-japanese-from-region', `speech-synth-english-from-buffer',
  and `speech-synth-english-from-region', `speech-synth-popup'.
* change default values of `speech-synth-intonation-default' and
  `speech-synth-postfilter-default'.
* add new variables of `speech-synth-vu-threshold-default' and
  `speech-synth-vu-threshold' to specify voiced/unvoiced threshold.
* change variable name of `speech-synth-buffer' to `speech-synth-buffer-name'.
* normalize sound volume when playing wav file of synthesized speech.