add label alignment settings

This fixes https://github.com/jacomyal/sigma.js/issues/460 and extends https://github.com/jacomyal/sigma.js/pull/291
- fix jslint errors
- update sigma.canvas.hovers.def.js for label alignment settings
- when zoom in/out, adjust hovered circle size for the node accordingly
- when use "center" setting, the rectangle area will not be displayed (it seem unnecessary to draw the rectangle on the top of the circle)
- when use "inside" setting, default setting will be used if the width of the label is larger than the hovered circle.

@jacomyal please review this for https://github.com/jacomyal/sigma.js/pull/291