Get DeepFormants working again

* Minor syntax tweaks to make the code Python 3 compatible

* Fixes for various NumPy warnings/errors, either due to use of "float"
  where "int" is required, or domain errors on log functions

* Replaced the use of the obsolete Python-2-only scikits.talkbox
  library with a compatible LPC implementation from the Conch project

* Documentation update to indicate that an old version of "rnn" is
  required

* Invoke Lua scripts via "luajit" directly, instead of going through
  the "th" frontend (to reduce the dependency footprint)

formants.sh

@@ -4,12 +4,12 @@ if [ $# -eq 2 ]
 then
     tempfile=`mktemp -t txt`
     python extract_features.py $1 $tempfile
-    th load_estimation_model.lua $tempfile $2
+    luajit load_estimation_model.lua $tempfile $2
 elif [ $# -eq 4 ]
 then
     tempfile=`mktemp -t txt`
     python extract_features.py $1 $tempfile --begin $3 --end $4
-    th load_estimation_model.lua $tempfile $2
+    luajit load_estimation_model.lua $tempfile $2
 else
     echo "$0 wav_filename pred_csv_filename [begin_time end_time]"
 fi