implemented ggplot2 theme

.../java/com/xeiam/xchart/demo/charts/line/ExampleChart.java

...n/java/com/xeiam/xchart/demo/charts/line/LineChart02.java

...n/java/com/xeiam/xchart/demo/charts/line/LineChart03.java

...n/java/com/xeiam/xchart/demo/charts/line/LineChart04.java

...n/java/com/xeiam/xchart/demo/charts/line/LineChart05.java

...n/java/com/xeiam/xchart/demo/charts/line/LineChart06.java

...n/java/com/xeiam/xchart/demo/charts/line/LineChart07.java

...n/java/com/xeiam/xchart/demo/charts/line/LineChart08.java

...n/java/com/xeiam/xchart/demo/charts/line/LineChart09.java

...n/java/com/xeiam/xchart/demo/charts/line/LineChart10.java

...n/java/com/xeiam/xchart/demo/charts/line/LineChart11.java

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

...c/main/java/com/xeiam/xchart/internal/chartpart/Axis.java

...a/com/xeiam/xchart/internal/chartpart/AxisTickLabels.java

...va/com/xeiam/xchart/internal/chartpart/AxisTickMarks.java

.../java/com/xeiam/xchart/internal/chartpart/ChartTitle.java

...main/java/com/xeiam/xchart/internal/chartpart/Legend.java

...java/com/xeiam/xchart/internal/chartpart/PlotSurface.java

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

...rt/src/main/java/com/xeiam/xchart/style/StyleManager.java