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).
I made `benchmarks` a module, so it gets compiled to Dart when running `gulp build`. 

That's how it works right now:

1. tracuer compiles `modules`. 
2. benchpress compiles `build/js/benchmarks`. 

```
modules -> build/js
build/js/benchmarks-> benchpress-build
```

So you have one extra folder: `benchpress-build`. As a result, separate clean and serve tasks are required. We can, of course, change the build file, so that benchrpess puts the files into `build/benchpress-build`. But I think it may be confusing. What do you think?