Standardize output and use standard FS functions

This pull request tries to improve the output in both normal and verbose mode. It changes the normal output from

```
Running "instrument" task
OK
OK
OK
OK
OK
OK
OK
OK
OK
OK
OK

Done, without errors.
```

to something like

```
Running "instrument" task
Instrumented 2 files (skipped 9 files)

Done, without errors.
```

It does this by using more standard Grunt filesystem functions (`grunt.file.read`, `grunt.file.write`) and by outputting less by default.