Remove Dygraph.log and all its kin

It's unclear what advantage this has over `console.log` and family, and it certainly has its disadvantages (extra code, less standard logging, etc.).

The polyfill lets us confidently include `console.warn` in our code, regardless of which browser is interpreting it.

This shrinks `dygraph-combined.js` from 121935→121712 bytes. 223 big bytes!

Fixes #454.