multiple_assign

First cut at fixing Wrong transformation on sequencial assign #34
- when we see an assign, check if the right node is an assign that was converted. if so, move the right assign above us, then convert the value to be a getter for that property we assigned to.
    
The AST structure that I'm returning seems to work, but isn't technically correct. Really we want a series of statements, but I couldn't figure out how to structure it so uglify didn't complain.