Add Line Shapes example to docs

- fig : https://plot.ly/~etpinard/436/linear-spline-vhv-hvh-vh-hv/
- inspired by : https://plot.ly/~alex/315/new-line-shape-options/
change example header, set bigger legend font in plot
Merge pull request #17 from plotly/line-shapes

Add Line Shapes example to docs