Opt tests

Adds tests for optAUC functions and optRMSE functions. Changes safeOptAUC and greedOptAUC to issue a warning and a message respectively in case of optimization underperforming a component model. In the case of safeOptAUC, the best model is weighted 1 and returned, in greedOptAUC the best optimization is returned with a message. 

Basic tests for greedOptRMSE and ability to issue a message in degenerate cases. However, no tests for degenerate cases are included because I couldn't generate any data to do this. 

Closes #75. 
Could we do `importFrom caTools colAUC` and `stopper <- max(colAUC(X, Y))` to keep the style consistent with other functions?