Pr/axis

### Features
* Pure Axes
* All `d3.svg.axis` functions are implemented that change the axis

### Notes
* Most of the `axis` functions can take no parameters and will spit out the current value or the default (e.g., `.ticks() => 10`). **None** of these are implemented. I'm not sure how necessary they are: I don't think any of them give meaningful or hard to compute information. Moreover, I don't know when they're actually used.
* The Axis implementation is in `D3/SVG/Axis.elm`. There is a new directory, it mirrors the D3js structure. D3 has a lower case naming scheme but `elm-d3` is natural case. Do you feel strongly about `SVG`, `Svg`, or `svg`?

### What next?
Once this gets merged in, I have my bar graph example that I'd like to get in. It could be a good resource for someone who wants to use `Scale` and `Axis`.