Pass (row, col) to valueFormatter

It's a little crazy that this information isn't already passed through to these callbacks. See the SO question linked to in the issue below. I'd like to include this in a 1.1.1 minor update.

Fixes #520 
I moved explanations of the parameters into a `params` section. Hopefully it's clearer now.

re: reordering methods, this was actually a documentation bug, it's not a behavior change. The `seriesName` parameter was always being passed to the formatter, but it wasn't documented.

If I were going to change how the dygraph object is passed to callbacks, I'd never pass it at all. `this` is the way to get it! :)