[BACK_INCOMPAT] `fabric.Text.textShadow`was removed and `fabric.BaseBrush` shadow properties removed (new `fabric.BaseBrush.shadow`)

[BACK_INCOMPAT] `fabric.Text#textShadow` has been removed - new `fabric.Text.shadow` property (type of fabric.Shadow)
[BACK_INCOMPAT] `fabric.BaseBrush`shadow properties are combined into one property => `fabric.BaseBrush.shadow`(shadowColor, shadowBlur, shadowOffsetX, shadowOffsetY no longer exist)
Removed fabric.Text.getText method => is generated with `fabric.util.createAccessors`
`fabric.Shadow` can now initialized with string or object (e.g. '10px 10px 5px rgb(0,0,255)', 'rgb(0,0,255) 10px 10px 5px', {color: 'gb(0,0,255)', offsetX: 10, offsetY: 10, blur: 5})
`fabric.Shadow.getShadow`to get css3 declaration of shadow (String)
`fabric.Object.set('shadow', value)`is now the same as `fabric.Shadow.setShadow(value)`
Add unit tests

Closes #819