Add sourceMap option to react-tools transform

Allow tools like grunt-react to include inline source maps in the
generated JavaScript. Browserify can then combine these source maps when
bundling everything together.

Usage:

```
var transform = require('react-tools').transform;
var output = transform(jsxContent, {
  sourceMap: true,
  sourceFilename: 'source.jsx'
});
```

The `output` will have an inline source map comment appended.

main.js

@@ -2,15 +2,31 @@
 
 var visitors = require('./vendor/fbtransform/visitors');
 var transform = require('jstransform').transform;
+var Buffer = require('buffer').Buffer;
 
 module.exports = {
-  transform: function(code, options) {
+  React: React,
-    var visitorList;
+  transform: function(input, options) {
-    if (options && options.harmony) {
+    options = options || {};
-      visitorList = visitors.getAllVisitors();
+    var result = transform(visitors.react, input, options);
-    } else {
+    var output = result.code;
-      visitorList = visitors.transformVisitors.react;
+    if (options.sourceMap) {
+      var map = inlineSourceMap(
+        result.sourceMap,
+        input,
+        options.sourceFilename
+      );
+      output += '\n' + map;
     }
-    return transform(visitorList, code).code;
+    return output;
   }
 };
+
+function inlineSourceMap(sourceMap, sourceCode, sourceFilename) {
+  var json = sourceMap.toJSON();
+  json.sources = [ sourceFilename ];
+  json.sourcesContent = [ sourceCode ];
+  var base64 = Buffer(JSON.stringify(json)).toString('base64');
+  return '//# sourceMappingURL=data:application/json;base64,' +
+         base64;
+}