Added rectangles and Ellipse to graph nodes. Code modified to view saved nodegraph.svg in image-viewer.

**Now graph will appear as follows** : 
 - Main node will appear as central ellipse with dark color. 
 - Nodes related to central main node will appear as ellipse with faint color.
 - Rest nodes will appear in rectangle.

**Also now saved graphname.svg can be viewed in (SVG format supported) image-viewer with all it's elements**.
 - Previously redundant svg arrow elements was getting produced.
 - That's why downloaded/saved svg file was not showing arrows in Image Viewer( not Inkscape ).
 - Now d3 code is modified to produce correct and standardized svg data.