Added 2 features: additional guiding lines & centered resize mode

It's possible to add additional lines to the selection area by specifying an array of positions (between 0 and 1). So if you want to add lines for the "Rule of Thirds" for instance, the options would be { createHGuides : [1/3, 2/3], createVGuides: [1/3, 2/3] }.

The second commit contains a change to the selection resizing. When the option 'resizeCentered' is set to true, dragging one of the edges 'n','w','s','e' results in resizing the adjacent two edge equally (so the selection will be resized on the left and right if you drag down the south edge). When starting a new selection, the selection mode automatically switches to 'edge-resizing' if you drag the second corner within the center third of each edge, otherwise it will use 'corner-resizing'.