ENH: Adding plot_drop_log() viz function

I use this after my preprocessing steps to see if there are channels I might have missed that are bad, or if my continuous + blink SSPs are working properly, etc. I figured it might be useful for others, as well. Here's the new output for the example function:

![image](https://f.cloud.github.com/assets/2365790/133008/5d5dc202-709a-11e2-83d6-ae14c344a0ec.png)