Add Start and End angle to circle.

Add option to circle for start and end angle, to draw an ARC.
in radiants, not in degree to avoid writing a setter and getter that does the conversion.
Internally is better radiants i think.

as per request of issue #1612

I modified the render, the initialize method, toObject and toSvg methods.
Did i forget something?
Bounding box is the original of the circle, i think it makes sense to leave how it is.

Canvas:
![image](https://cloud.githubusercontent.com/assets/1194048/4313024/1c6ed540-3ec2-11e4-9a14-4fba35907b42.png)

svg export
![image](https://cloud.githubusercontent.com/assets/1194048/4313035/4576ed38-3ec2-11e4-93bb-aa619961ce22.png)


resulting svg
```xml
<?xml version="1.0" encoding="UTF-8" standalone="no" ?><!DOCTYPE svg PUBLIC "-//W3C//DTD SVG 1.1//EN" "http://www.w3.org/Graphics/SVG/1.1/DTD/svg11.dtd">
<svg xmlns="http://www.w3.org/2000/svg" xmlns:xlink="http://www.w3.org/1999/xlink" version="1.1" width="900" height="500" xml:space="preserve"><desc>Created with Fabric.js 1.4.11</desc><defs></defs><path d="M 21.213203435596427 21.213203435596423 A 30 30 0 0 1  -21.213203435596423 21.213203435596427" style="stroke: #00FF00; stroke-width: 1; stroke-dasharray: ; stroke-linecap: butt; stroke-linejoin: miter; stroke-miterlimit: 10; fill: #00FF00; fill-rule: source-over; opacity: 1;" transform="translate(445.74 232.61) rotate(51.79) scale(4.21 4.21) "/>
<path d="M -21.213203435596423 -21.213203435596427 A 30 30 0 0 1  21.213203435596427 -21.213203435596423" style="stroke: #FF0000; stroke-width: 1; stroke-dasharray: ; stroke-linecap: butt; stroke-linejoin: miter; stroke-miterlimit: 10; fill: #00FF00; fill-rule: source-over; opacity: 1;" transform="translate(305.74 242.61) rotate(51.79) scale(4.21 4.21) "/>
<path d="M -21.213203435596423 21.213203435596427 A 30 30 0 0 1  -21.21320343559643 -21.213203435596423" style="stroke: #0000FF; stroke-width: 1; stroke-dasharray: ; stroke-linecap: butt; stroke-linejoin: miter; stroke-miterlimit: 10; fill: #00FF00; fill-rule: source-over; opacity: 1;" transform="translate(439.2 278.3) rotate(51.79) scale(3.86 4.21) matrix(-1 0 0 1 0 0)  "/>
<path d="M 21.213203435596427 -21.213203435596423 A 30 30 0 0 1  21.213203435596427 21.213203435596423" style="stroke: #00FFFF; stroke-width: 1; stroke-dasharray: ; stroke-linecap: butt; stroke-linejoin: miter; stroke-miterlimit: 10; fill: #00FF00; fill-rule: source-over; opacity: 1;" transform="translate(291.74 316.61) rotate(51.79) scale(4.21 4.21) matrix(-1 0 0 1 0 0)  "/>
<path d="M 21.213203435596427 21.213203435596423 A 30 30 0 0 1  -21.213203435596423 21.213203435596427" style="stroke: #00FF00; stroke-width: 1; stroke-dasharray: ; stroke-linecap: butt; stroke-linejoin: miter; stroke-miterlimit: 10; fill: #00FF00; fill-rule: source-over; opacity: 1;" transform="translate(174.5 130.5) "/>
<path d="M -21.213203435596423 -21.213203435596427 A 30 30 0 0 1  21.213203435596427 -21.213203435596423" style="stroke: #FF0000; stroke-width: 1; stroke-dasharray: ; stroke-linecap: butt; stroke-linejoin: miter; stroke-miterlimit: 10; fill: #00FF00; fill-rule: source-over; opacity: 1;" transform="translate(174.5 130.5) "/>
<path d="M -21.213203435596423 21.213203435596427 A 30 30 0 0 1  -21.21320343559643 -21.213203435596423" style="stroke: #0000FF; stroke-width: 1; stroke-dasharray: ; stroke-linecap: butt; stroke-linejoin: miter; stroke-miterlimit: 10; fill: #00FF00; fill-rule: source-over; opacity: 1;" transform="translate(174.5 130.5) "/>
<path d="M 21.213203435596427 -21.213203435596423 A 30 30 0 0 1  21.213203435596427 21.213203435596423" style="stroke: #00FFFF; stroke-width: 1; stroke-dasharray: ; stroke-linecap: butt; stroke-linejoin: miter; stroke-miterlimit: 10; fill: #00FF00; fill-rule: source-over; opacity: 1;" transform="translate(174.5 130.5) "/>
</svg>
```