Fix for bounding box of path, opacity of group, some transform

This PR adds:
Path Class
Correct calculation of bounding box for paths ( with bezier and arc commands ).
Removed render method in path class to use the standard obejct one
Removed 1 ctx.transform in path _render method

Object Class
Correct rendering of opacity for groups, added setOpacity method.
Removed _Transform method, it was confusing in the render logic. "transform" method is enough
TransformMatrix on single objects are not supported yet, but behaves a little better, at least the object now follow mouse movements and doesn't go in different direction.

Text class:
Made some changes to adapt to object class. 

arc.js
Add functions to calculate the bounding box.