new feature: save as a high res png by specifying DPI

.gitignore

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

...o/src/main/java/com/xeiam/xchart/standalone/Example1.java

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

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

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

...n/java/com/xeiam/xchart/internal/chartpart/AxisTitle.java

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

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

...c/main/java/com/xeiam/xchart/internal/markers/Circle.java

.../main/java/com/xeiam/xchart/internal/markers/Diamond.java

...c/main/java/com/xeiam/xchart/internal/markers/Marker.java

...c/main/java/com/xeiam/xchart/internal/markers/Square.java

.../java/com/xeiam/xchart/internal/markers/TriangleDown.java

...in/java/com/xeiam/xchart/internal/markers/TriangleUp.java