Y domain baseline

Currently, setting `axis.y2.min` to `0` would be ignored, as `isValue` returns `false` if given `0` - but what if you want to force it to be `0`? This results to the following, marking the axis start at `-5`:

![screen shot 2014-11-14 at 14 29 39](https://cloud.githubusercontent.com/assets/2316326/5047198/b97c0a7c-6c0a-11e4-8a3e-57f01c83703f.png)

As such, I am introducing an internal function, `isNumber`, which checks if a value is a number or not; if it is, then we can let `y` be `0`. And here is the result. with the axis starting at `0` and marked as `0`:

![screen shot 2014-11-14 at 14 31 07](https://cloud.githubusercontent.com/assets/2316326/5047250/4309628a-6c0b-11e4-9c1f-1a1d492c35f8.png)

Let me know what you think!