more work on axis tick calculators, making abstract

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

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

...rnal/chartpart/axistickcalculator/AxisTickCalculator.java

.../chartpart/axistickcalculator/DateAxisTickCalculator.java

...art/axistickcalculator/LogarithmicAxisTickCalculator.java

...hartpart/axistickcalculator/NumberAxisTickCalculator.java

...nternal/chartpart/axistickcalculator/NumberFormatter.java

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

...ava/com/xeiam/xchart/unit/DateAxisTickCalculatorTest.java

.../com/xeiam/xchart/unit/DecimalAxisTickCalculatorTest.java

...c/test/java/com/xeiam/xchart/unit/ValueFormatterTest.java