Add curve & arrow edge rendering

In previous library version there was ability to render edges both curve with arrow head. I have restored this feature for the recent version. The edge type can be chosen by specifying 2 parameters: `edge.type` & `edge.head`. At the moment there are four edge types:
* straight
``` javascript
edge.type = 'def' || '' || undefined
edge.head = 'def' || '' || undefined
```
* curve
``` javascript
edge.type = 'curve'
edge.head = 'def' || '' || undefined
```
* straight with arrow head
``` javascript
edge.type = 'def' || '' || undefined
edge.head = 'arrow'
```
* curve with arrow head
``` javascript
edge.type = 'curve'
edge.head = 'arrow'
```