code for issue #3749 - change the format of displayed results in classification_report

Made a simple enhancement for issue #3749, adding digits to the optional arguments to the classification_report function with default value of 2 (matches previous hard coded value). Allows user to print out the floating values with whatever precision they want in the format.

No unit test added at this time, I will work on that once I've figured out the testing setup using Nose. Any suggestions for test would be welcome.