WIP: Berksfile support

Reworked Berksfile support. There is one more thing to solve to properly use Berkshelfe we should not use any local cookbooks and only use cookbooks from Berksfile. Otherwise I use temporary directory for berks vendor if it's not said other way.

1) README
2) tests 

Will be done after initial approval.
I would put all the rest inside an `else`. So:

```python
try:
    env.berksfile = ...
except
   ...
else:
   do the rest
```