Added a custom point renderer

This allows us to draw points at arbitrary sizes.
Merge branch 'master' of https://github.com/feerrenrut/windcast into addCustomVertices
Merge branch 'master' of https://github.com/feerrenrut/windcast into addCustomVertices
custom point renderer now shows an arrow image at each data location.
image is rotated to point in the direction that the wind is going
Merge pull request #4 from feerrenrut/addCustomVertices

Add custom vertices