Changed default training function to randomForest.
Fixed documentation of FOS-R custom class index annotation.
ref FOS-7 fos-r: Updated fos-r missing value handling

Summary:
Now handles null's as missing values and converts them accordingly:
	When training it is converted to "?" because the dataset is saved in the ARFF format.
	When scoring it is converted to "NA" because that is the missing value constant in R.
The default training function is SVM which by default uses na.action=na.drop
When using randomForest the default na.action is na.fail, so I explicitly added the na.drop parameter.

Also added tests that cover both training and scoring with missing values.

Test Plan: Modified existing tests to include missing values

Reviewers: ricardo.ferreira, antonio.alegria

Reviewed By: antonio.alegria

Subscribers: deag