Initial work on stats.
Stats collectors.

* Collectors collect an array of stat values.
* The Stats object totals these values.
Break DiffFile into Diff and File.
Update README with new example code.
Collector can now either return an array of values or a single value.
Make line number an optional parameter when initializing a new line.

* Instead set the line number with line_number=
* LineNumberCalculation now sets the line number before incrementing.
Simplify Line class determination.

* Remove the need for Line::Change.
* Remove some duplication.
Fixing HunkCollector -> HunkStatsCollector.
Refactor Stats collection.
Update README.
Remove unused code.
Merge pull request #4 from anolson/stats

Stats support.