added concat grunt task and dependency
Fixed canvas renderers when label is missing

From now, when a label is missing:
 - It is just not rendered by sigma.canvas.labels.def
 - Only the shadow will be displayed by sigma.canvas.hovers.def, no more background.
Clarification on MDN Function#bind polyfill legal status

MSDN => MDN : I killed people for less than that ;-)
The code is in [the public domain](https://developer.mozilla.org/en-US/docs/Project:MDN/About)

> Code samples added on or after August 20, 2010 are in the public domain.

(wiki page started on August 26 2010)
Merge remote-tracking branch 'origin/pr/146' into draft-v1.0.0
Merge remote-tracking branch 'origin/pr/145' into draft-v1.0.0