Merge legend and poly.line bugfixes into master

[djvill]

Fixed the following bugs with legend and poly.line:

1. When defining both vowel and group (i.e., vowel!=NULL & group!=NULL), setting plot.tokens=TRUE and plot.means=FALSE, and coloring by group, the wrong colors show up in the legend (regardless of values of var.sty.by or pretty); the right colors show up under the same conditions if coloring by vowel.
2. There's a similar issue for poly.line=TRUE: if the style variable is vowels and the color variable is something else (i.e., the poly.line connects vowels of the same color), all linetypes will always be the first value of poly.args$lty/exargs$lty or, if both are NULL, solid (lty=1).
3. Under the same circumstances, the wrong linetypes show up in the legend
4. When poly.line=TRUE, the color variable is vowels, and the style variable is something else, colored boxes don't show up next to vowels in the legend.