feat(bench press): allow multiple reporters, metrics and driver extensions.

modules/benchpress/benchpress.js

@@ -14,5 +14,7 @@ export { IOsDriverExtension } from './src/webdriver/ios_driver_extension';
 export { Runner } from './src/runner';
 export { Options } from './src/sample_options';
 export { MeasureValues } from './src/measure_values';
+export { MultiMetric } from './src/metric/multi_metric';
+export { MultiReporter } from './src/reporter/multi_reporter';
 
 export { bind, Injector, OpaqueToken } from 'angular2/di';