Allow to specify an objectToExportTo for goog.exportSymbol

This allows users to build ol3 without anything exposed in the
global namespace. This can e.g. be useful for creating an ol3
AMD module, by simply using a build configuration with
"define('ol',function(){var o={};%output%return o.ol;});" as
"output_wrapper".

tasks/build.js

@@ -27,6 +27,10 @@ function assertValidConfig(config, callback) {
       callback(new Error('Config missing "exports" array'));
       return;
     }
+    if (config.namespace && typeof config.namespace !== 'string') {
+      callback(new Error('Config "namespace" must be a string'));
+      return;
+    }
     if (config.compile && typeof config.compile !== 'object') {
       callback(new Error('Config "compile" must be an object'));
       return;
@@ -182,7 +186,7 @@ function build(config, paths, callback) {
 function main(config, callback) {
   async.waterfall([
     assertValidConfig.bind(null, config),
-    generateExports.bind(null, config.exports),
+    generateExports.bind(null, config.exports, config.namespace),
     getDependencies.bind(null, config.src),
     build.bind(null, config)
   ], callback);