Legend entries, label positions

* Legends: If there is an empty legend string, legends/plots became assigned incorrectlly. The decisions to "forget the plot" in drawLine / draw* and to add a legend entry in src/matlab2tikz.m:712 were not coherent.
* Axis label: Axis ticks and labels should be at the same position (left/right, top/bottom). Only the axis position was set as in Matlab, but the label was not. A simple plot with two y axes created the two labels at the left, on top of each other:
`x=0:0.1:2*pi; [AX] = plotyy(x, sin(x), x, cos(x)); set(get(AX(1),'Ylabel'),'String','sin'); set(get(AX(2),'Ylabel'),'String','cos');`