default strength for collide is 1

fixes #127

README.md

@@ -211,7 +211,7 @@ The radius accessor is invoked for each [node](#simulation_nodes) in the simulat
 
 <a name="collide_strength" href="#collide_strength">#</a> <i>collide</i>.<b>strength</b>([<i>strength</i>]) [<>](https://github.com/d3/d3-force/blob/master/src/collide.js#L82 "Source")
 
-If *strength* is specified, sets the force strength to the specified number in the range [0,1] and returns this force. If *strength* is not specified, returns the current strength which defaults to 0.7.
+If *strength* is specified, sets the force strength to the specified number in the range [0,1] and returns this force. If *strength* is not specified, returns the current strength which defaults to 1.
 
 Overlapping nodes are resolved through iterative relaxation. For each node, the other nodes that are anticipated to overlap at the next tick (using the anticipated positions ⟨*x* + *vx*,*y* + *vy*⟩) are determined; the node’s velocity is then modified to push the node out of each overlapping node. The change in velocity is dampened by the force’s strength such that the resolution of simultaneous overlaps can be blended together to find a stable solution.