WIP - Add linting support

Hello,

As discussed in hapijs/discuss#22, I'm trying to implement linting in lab.
This is a WIP and shouldn't be merged right now as tests are not even done.
So far I only have support for eslint, I included @geek's rules but lab itself is full of errors so it'll need tweaking.
It will automatically detect eslint settings at the root of the project and if none use what should be our default rules.
The possibility to have multiple linters is deliberate and probably wanted.
Console output was faster to implement but others will follow.
I'd like feedback about my approach.

Thanks.
Might be simpler to check:

```
Hoek.assert(internals.linters.indexOf(linter) !== -1, 'Linter ' + linter + ' is unknown.');
```