Added edge arrow rendering

Features
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- Arrowhead location setting can be specified globally in sigma instance drawingProperties (via `defaultEdgeArrow` property) and/or each individual edge (via `arrow` property): **'target', 'source', 'both', 'none'**  _[default]_
- Arrowhead color/size/orientation accommodates edge color/size/type (curved, line)
- Arrowhead tips are drawn at node border (instead of overlapping with nodes)
- Arrowhead display size scales with zoom

Caveats
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- Assumes circular nodes. To add support for future node shapes, see function `applyArrow()` in plotter.js

Demos
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- http://hansifer.com/sigmaArrowDemo1.htm
- http://hansifer.com/sigmaArrowDemo2.htm

![image](https://f.cloud.github.com/assets/1163334/502805/29f41e30-bcbb-11e2-838b-0874f8daa3a1.png)