Return target on mouse:down (even for unselectable objects)

canvas 'mouse:down' event only returns selectable objects.
Clear selection for unselectable object

Now objects are found even they are unselectable - expand _shouldClearSelection for unselectable objects.
Don't call findTarget() - new in parameter target.
findTarget now retuns unselectable objects
Bugfix object.opacity == 0 in svg output

If object.opacity = 0 the svg output sets opacity to 1.
Merge pull request #572 from Kienz/patch-2

Bugfix object.opacity == 0 in svg output
- lockUniScaling now can't be overridden with shiftKey or uniScaleTransform property
- Pass event to object 'rotating'
- some refactoring
Merge pull request #576 from Kienz/lockUniScaling

 lockUniScaling now can't be overridden with shiftKey or uniScaleTransform property
Merge pull request #568 from Kienz/patch-1

Return unselectable objects in canvas 'mouse:down' event
Cap jsdom at 0.5.6
Optimize _initDimensions to not create off-screen canvas every time. Closes #564
Build distribution
Add support for pattern offsets (via fabric.Pattern#offsetX/offsetY). Version 1.1.8
Fix doc