Update the highcharts-ng directive to support the yAxis as well as xAxis...

This is just andrewt3000's pull request from here https://github.com/pablojim/highcharts-ng/pull/19 refactored to remove code duplication as you had requested.  I didn't think it was possible for me to submit an update to his pull request so I just made a new one.  I've tested locally to ensure that the yAxis properties are working.