Added line chart prop for horiz only grid lines

Added new boolean property: onlyHorizontalGridLines to toggle between
full grid and horizontal only grid lines
Added bar chart prop for horiz only grid lines

Boolean property "onlyHorizontalGridLines" added to show only horizontal
grid lines when scaleShowGridLines is true
Fix for issue#149

scaleShowGridLines set to false does not remove vertical gird lines in
bar chart
Merge branch 'onlyhorizontal'

Conflicts:
	Chart.js