Make browserify case work both for npm and bower.

Author: vprus

src/footer.js

@@ -3,10 +3,17 @@ return dc;}
     if(typeof define === "function" && define.amd) {
         define(["d3", "crossfilter"], _dc);
     } else if(typeof module === "object" && module.exports) {
-        // When using window global, window.crossfilter is a function
-        // When using require, the value will be an object with 'crossfilter'
-        // field, so we need to access it here.
-        module.exports = _dc(require('d3'), require('crossfilter').crossfilter);
+        var _d3 = require('d3');
+        var _crossfilter = require('crossfilter');
+        // When using npm + browserify, 'crossfilter' is a function,
+        // since package.json specifies index.js as main function, and it
+        // does special handling. When using bower + browserify,
+        // there's no main in bower.json (in fact, there's no bower.json),
+        // so we need to fix it.
+        if (typeof _crossfilter !== "function") {
+            _crossfilter = _crossfilter.crossfilter;
+        }
+        module.exports = _dc(_d3, _crossfilter);
     } else {
         this.dc = _dc(d3, crossfilter);
     }