Fix issue, when setting brush width as string

There was a bug( https://github.com/kangax/fabric.js/issues/448 ) when
setting freeDrawingBrush.width=string. Added "setWidth" method to
base_brush.class witch does "parseInt" on given width, and then sets it.
Maybe it's not a bug, but I've wasted an hour or two, because of this,
decide if it is worth the changes(or should just js programmers be more
careful).