Specify generic image in javascript

Also, had to turn off `avoidOverlap` for now. This option only has a
real impact when physics are turned off and new nodes are brought into
the graph visualization. They will now overlap. We can presumably turn
this option back on when this bug is fixed:
almende/vis#1625

dist/ml-visjs-graph-ng.js

@@ -250,6 +250,8 @@
         borderWidth: 2,
         shadow: true,
         borderWidthSelected: 6,
+        shape: 'circularImage',
+        image: 'bower_components/ml-visjs-graph-ng/dist/images/generic.png',
         color: {
           background: 'white'
         },
@@ -285,7 +287,7 @@
           springLength: 200,
           springConstant: 0.04,
           damping: 0.5,
-          avoidOverlap: 1
+          avoidOverlap: 0
         },
 
         // built-in default
@@ -304,7 +306,7 @@
           springLength: 100,
           springConstant: 0.08,
           damping: 0.4,
-          avoidOverlap: 1
+          avoidOverlap: 0
         },
 
         // built-in default

src/visjs-graph/visjs-graph.controller.js

@@ -129,6 +129,8 @@
         borderWidth: 2,
         shadow: true,
         borderWidthSelected: 6,
+        shape: 'circularImage',
+        image: 'bower_components/ml-visjs-graph-ng/dist/images/generic.png',
         color: {
           background: 'white'
         },
@@ -164,7 +166,7 @@
           springLength: 200,
           springConstant: 0.04,
           damping: 0.5,
-          avoidOverlap: 1
+          avoidOverlap: 0
         },
 
         // built-in default
@@ -183,7 +185,7 @@
           springLength: 100,
           springConstant: 0.08,
           damping: 0.4,
-          avoidOverlap: 1
+          avoidOverlap: 0
         },
 
         // built-in default