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Python 3 support fixes (tested with 3.4).
Changed index of `<BOS>` and `<EOS>` tokens to be \\x03 `STX` and \\x04 `ETX`
since the input here is much closer to TTY behavior than traditional
word vocabularies (in which 1 and 2 are usually reserved for `<S>` and
`</S>` respectively).
Added some doc support tools to a doc extra.