added Theme interface

.../src/main/java/com/xeiam/xchart/demo/charts/Example1.java

...src/main/java/com/xeiam/xchart/demo/charts/Example10.java

.../src/main/java/com/xeiam/xchart/demo/charts/Example2.java

.../src/main/java/com/xeiam/xchart/demo/charts/Example4.java

.../src/main/java/com/xeiam/xchart/demo/charts/Example8.java

.../src/main/java/com/xeiam/xchart/demo/charts/Example9.java

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

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

...in/java/com/xeiam/xchart/internal/chartpart/AxisPair.java

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

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

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

...chart/internal/misc/SeriesColorMarkerLineStyleCycler.java

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

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

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

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

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

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

xchart/src/main/java/com/xeiam/xchart/style/theme/Theme.java