Issue #2: Remove dependency on servlet jar when saving a png

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

src/main/java/com/xeiam/xchart/io/ServletEncoder.java

...st/java/com/xeiam/xchart/example/ChartServletExample.java

src/test/java/com/xeiam/xchart/example/Example1.java