feat(benchmark): add a simple benchmark for the di module

It adds the following tasks to the gulpfile:

1. `benchmarks/clean` // removes the compiled files
2. `benchmarks/build` // compiles all the files in `benchmarks` and creates a benchpress build
3. `benchmarks/run` // fires up a server

It also includes a very simple benchmark. It is not very useful and just serves as an example.

Extra notes:
1. Benchmarks run against the build folder. Right now everything is compiled with type checks enabled. We need to have another task (or maybe a flag) that compiles everything in production mode (without type checks).
@vojtajina added the es6 module loader to the project, so the dependent files can be loaded on demand. For this to work you need a script that calls `System.import('di/benchmark')` and then registers the benchmark steps at benchpress. As this uses promises we need to change benchpress to support manual bootstrap or make it support ES6 module directly (using `System.load`). See https://github.com/angular/benchpress/issues/13

See https://github.com/angular/angular/blob/master/test-main.js for how karma is configured.