Add option to exclude zero as default Y-axis value.
Merge branch 'exclZero' of https://github.com/cjhaug/plot-plugin into cjhaug-exclZero

Conflicts:
	src/main/java/hudson/plugins/plot/MatrixPlotPublisher.java
	src/main/java/hudson/plugins/plot/Plot.java
Merge branch 'cjhaug-exclZero'
Refactoring on pull request #11