Added test for random forest regression

[ankane]

Added a test for #13. To generate the PMML file, I used:

library(randomForest)
library(pmml)

set.seed(1)

model <- randomForest(dist ~ speed, cars, ntree=3)

saveXML(pmml(model), "cars_rf.pmml")

print(predict(model, data.frame(speed=10)))

[coveralls]

Coverage decreased (-0.2%) to 99.359% when pulling 7dfa07c on ankane:master into 924ed6a on asafschers:master.

[coveralls]

Coverage decreased (-0.2%) to 99.359% when pulling 7dfa07c on ankane:master into 924ed6a on asafschers:master.