Update README to mention Festival

Changes the opening blurb to include it with the other services.
Much improve voice list detection for eSpeak

This changes the official driver for the eSpeak voices to the long names
given in a --voices output, now combining with --voices=mb (which
provides MBROLA voices, distinct from the regular eSpeak voices).

The language codes are still supported, and will be remapped
automatically to the longer voice names.
Make Google's voice attribute more forgiving

This allows the user to now enter full language names and still get the
desired result.
Additional improvements to voice lookups

Generating the voice_lookup map object is now deferred to the options()
call (if possible) and more aliases are allowed.