Added `--blank` flag to generate project skeletons (no sample content)

I added the `--blank` flag to generate only the directory tree and the minimum required code (e.g. imports, main(), basic html page) without any sample content, this would let the user save some time :)
Feel free to replace "--blank" with something more meaningful.

Tests run fine, I added 'em to `drone/run.sh`, even if drone.io tests break when trying to run polymer app's build.dart[1].

I also removed an unused import, `dart:async`, from `tool/hop_runner.dart`.

Let me know what you think :+1: 

Claudio
[1] https://drone.io/github.com/damondouglas/toolshed.dart/24