Project: svg-chart
Task (moderate): In `common/Chart.js`, when `options.axes` is true or not defined:
- append g element (class `axes axes-x`) inside `svg` and align it to the bottom
- append g element (class `axes axes-y`) inside `svg` and align it to the left
- append axis-x line (class `axis axis-x`, color gray) inside `.axes-x` and align it to the top
- append axis-y line (class `axis axis-y`, color gray) inside `.axes-y` and align it to the right

In `common/LineChart.js`, when `options.axes` is true or not defined:
- place `.datasets` to the right side of `.axis-y` and above `.axis-x`
- reduce `.datasets` lines width and height, and make `.datasets`, `.axes-x` and `.axes-y` fill the entire `svg` space