repull: Add three regression models into HLearn-classification

I cleaned some trash and keep important four changed files

according to 
http://mathworld.wolfram.com/LeastSquaresFittingExponential.html#eqn5
origin Exponential fit gives greater weights to small y values so, in order to weight the points equally, it is often better to minimize the function 
ModifiedExponential is tuned for that