Added test for integer overflow in Curve.rocArea() from issue15.
Cleaned up comments for testRocAreaIntegerOverflow().
Updated unit test for issue15.

* Added check for mannWhitneyU() for best and worst cases.
* Edited comments.
Fixed integer overflow in rocArea() and mannWhitneyU().

* Changed totalPositive and totalNegative products to cast to
  double before multiplying so floating point multiplication
  is done to avoid integer overflows.
Updated comments and fixed whitespace.

* Fixed whitespace to align with conventions in
  CurveTest.testRocAreaIntegerOverflow().
* Added more detailed comments for issue15 fix in Curve.rocArea()
  and Curve.mannWhitneyU().
* Fixed other whitespace issues with integer overflow fix.
Changed scale test for testRocAreaIntegerOverflow() to match our supported scale.

* Changed number of positive and number of negative examples from 2^17 to 2^20.
  This test now matches the comment in Curve. rocArea() that we support up to
  1 million examples. Run time for the test is still <0.1 seconds.
Converted tabs to spaces.
Updated copyright notice dates.
Added statement of supported scale to README.
Updated copyright year on README.
Merge pull request #18 from kboyd/issue15_v2

Issue15 (integer overflow) fix version 2