Improve documentation

It is very hard now to understand how to use this tool.
It would be great, if you add some HOW-TOs, examples, typical scenarios and so on.

e.g I fund out that I can't define tasks in camelCase like that:

```
@Task()
someTask() => ...
...
> grind someTask
Error, no task found: 'someTask'.
```

but that works fine:

```
@Task()
sometask() => ...
...
> grind sometask
OK
```

And that is just a simple case that lies on the surface.