Fix not find "y_grid_lines" function and subsetting bug

Function y_grid_lines is not exported and is in plot_object$Env. chart*
functions called in add* are passed to "lenv" environment so y_grid_lines
cannot be found.

y_grid_lines function is defined in chart* by
y_grid_lines <- plot_object$Env$y_grid_lines.

For add*MA functions, x.tmp should not be passed by x.tmp[xsubset] because
when zoomChart is called, series used to calculate moving averages will
change with the subset period if we pass x.tmp[xsubset] to x.tmp. Moving
avrage indicators are passed to ma by ma <- *MA(x.tmp, ...)[xsubset] so
x.tmp should always be the initial series and should not change with the
subset period.