Add Caret Earth Method

[antoinecarme]

for multi-class classification and regression (MARS equivalent for R)

As described in issue #2

https://cran.r-project.org/web/packages/earth/index.html
https://cran.r-project.org/web/packages/earth/earth.pdf
https://github.com/cran/earth
https://github.com/topepo/caret/blob/master/models/files/earth.R

[antoinecarme]

Added earth method classification and regression models support