made API prefer to accept a Collection of Number objects for the axis data

Sample_Chart.png

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

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

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

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

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

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

src/test/java/com/xeiam/xchart/example/Example2.java

src/test/java/com/xeiam/xchart/example/Example3.java

src/test/java/com/xeiam/xchart/example/Example4.java

src/test/java/com/xeiam/xchart/example/Example5.java

src/test/java/com/xeiam/xchart/example/Example6.java

src/test/java/com/xeiam/xchart/example/Example7.java

...test/java/com/xeiam/xchart/unit/TestNumberCollection.java