Adding variable labels to graph axes and legend when available

These changes make binscatter use the variable label, instead of the variable name, to label the axes and the legend entries. If the variable label is empty, it uses the variable name instead. This works with "by" and with multiple y-variables. 

I've tested it briefly using the examples given in the help file for binscatter, but I haven't had a chance to exhaustively test it yet. In particular, I don't know what happens when you add special characters (like quotes) to a variable label and then try this command.