Increase graph fluidity + update/refactor scaling code

Remove cursor circle outline since it was not updating correctly when
outside svg area.

Add functionality to scale node icons and links using slider. Also
rename slider from zoom to scale (was not a true zoom, objects stayed
in place).

Introduce new k-value heuristic used to pick appropriate charge and
gravity values based on graph density.
Graphs generated based on k-value look and feel much more fluid than
before. Idea by mbostock on Stack Overflow.

Misc. refactors
  -function resize() moved to be next to slider javascript
  -added BASE_VALUE constant for slider default and for
calculating scaling factor
  -force layout initialization in two separate places combined