Implement strokeOpacity and fillOpacity

- There now 3 opacity properties: opacity (global), fillOpacity and strokeOpacity
- opacity affects fillOpacity and strokeOpacity (same as in svg)
- Remove stroke and strokeWidth from fabric.Text - defined in fabric.Object
- Doc additions
- Update unit tests

@kangax What do you think of this? Now you can control stroke's and fill's opacity separately or together with opacity property ("old" behavior).