Better strokeDashArray support + Fixes

- fabric.Text has now strokeDashArray support (only native support)
- fabric.Text.fill = null should now work
- Fix save/restore context in render methods => ctx.setLineDash affected drawBorder/drawControls
- Add strokeLineCap (default "butt"), strokeLineJoin (default "miter") and strokeMiterLimit (default 10)
- Add support for fabric.Object#fromElement for strokeDashArray (and other stroke properties)
- Add @default tag to properties (JSDoc 3)
- strokeDashArray now only works if stroke property is defined
- Add strokeLineCap (default "round"), strokeLineJoin (default "round") to fabric.BaseBrush
- Updated unit tests

Look at issue #107