Implemented calculation of zeroth PR point.

* Decided on how to calculate zeroth PR point (which is undefined due to
  division by zero): copy precision (vertical) value of first point to
  zeroth point creating a horizontal line that extends from the first
  defined point left to the vertical axis.
* Implemented above for methods 'prPoint' and 'rawPrPoints'.
* Updated corresponding tests.
Implemented (properly) PR points for plotting.

* Implemented generation of PR points appropriate for plotting that uses
  the closest linear underestimate of the actual PR curve without
  introducing samples.  This involves introducing some "lower-left"
  points where there are ties.
* Improved documentation on 'rocPoints' and 'prPoints'.
* Deleted obsolete methods: 'prPoints(int)' and 'rawPrPoints'.
* Updated unit tests accordingly.
* Updated copyright year of unit test file, which escaped the previous
  commit.
Implemented basic PR area calculation.

* Implemented basic PR area calculation based on rectangles and
  trapezoids that closely underestimates the actual PR curve.
* Added test for 'prArea'.
* Added methods for precision and recall at a given threshold because
  they are useful for 'prArea' and in general.
* Refactored 'prPoint' to use new methods.
Updated Javadoc.

* Updated Javadoc so that every method has a summary sentence.
Comment out incomplete weights functionality prior to release.
Made README.md authority for version number.

* Stated version number in README.
* The README is now the single, authoritative source for the version
  number.
* Makefile parses the version from the README.
Updated feature list in response to recent implementation.