Update jquery.flot.threshold.js

Displays the serie base color and the threshold color side by side in the legend.
When no treshold is color is set, default black color is used.

![image](https://f.cloud.github.com/assets/1426742/1255912/9f1b8f6e-2b8c-11e3-8af2-c03090a9b943.png)