Added composer file to allow easier deployment

Adding a composer.json file brings nvd3 into line with d3 and its use of
composer files. This will allow PHP and any projects using composer to
more easily integrate the nvd3 library like that do with d3.

composer.json

@@ -0,0 +1,32 @@
+{
+  "name": "novus/nvd3",
+  "description": "A reusable charting library written in d3.js",
+  "keywords": [
+    "nvd3",
+    "d3",
+    "chart",
+    "graph"
+  ],
+  "homepage": "https://github.com/novus/nvd3",
+  "license": "Apache-2.0",
+  "authors": [
+    {
+      "name": "Bob Monteverde"
+    },
+    {
+      "name": "Tyler Wolf"
+    },
+    {
+      "name": "Robin Hu"
+    },
+    {
+      "name": "Frank Shao"
+    },
+    {
+      "name": "liquidpele"
+    }
+  ],
+  "require": {
+    "mbostock/d3": "@stable"
+  }
+}