updated copyright date

src/main/java/com/xeiam/xchart/Axis.java

src/main/java/com/xeiam/xchart/AxisLine.java

src/main/java/com/xeiam/xchart/AxisPair.java

src/main/java/com/xeiam/xchart/AxisTick.java

src/main/java/com/xeiam/xchart/AxisTickLabels.java

src/main/java/com/xeiam/xchart/AxisTickMarks.java

src/main/java/com/xeiam/xchart/AxisTitle.java

src/main/java/com/xeiam/xchart/BitmapEncoder.java

src/main/java/com/xeiam/xchart/Chart.java

src/main/java/com/xeiam/xchart/ChartColor.java

src/main/java/com/xeiam/xchart/ChartLegend.java

src/main/java/com/xeiam/xchart/ChartTitle.java

src/main/java/com/xeiam/xchart/Plot.java

src/main/java/com/xeiam/xchart/PlotContent.java

src/main/java/com/xeiam/xchart/PlotSurface.java

src/main/java/com/xeiam/xchart/QuickChart.java

src/main/java/com/xeiam/xchart/interfaces/IChartPart.java

src/main/java/com/xeiam/xchart/series/Series.java

src/main/java/com/xeiam/xchart/series/SeriesColor.java

src/main/java/com/xeiam/xchart/series/SeriesLineStyle.java