WIP: Berksfile support

Aims at #166.  Very much WIP, need to write tests/formal documentation, wanted to get a sense whether this was the right direction.  (Wrote a bunch of tests for one approach that turned out not to work in parallel mode so backed them out.)

Workflow:

User updates the kitchen section of the littlechef.cfg:

```
[kitchen]
berksfile = Berksfile
berksfile_cookbooks_directory = berks-cookbooks
```

Before running any 'fix' commands littlechef does a berks vendor into that directory, removing it if it already exists.  It does not remove the directory if the directory is newer than the Berksfile.

Had to do this in the `node` call because anything later runs into parallel execution issues, we only want to run berks vendor in single threaded mode

Still TODO:
* check for berkshelf 3 when config is specified, die if not there (not sure how much of this would work for berkshelf 2 as I've only ever used 3)
* check for existence of Berksfile
* berks vendor fails = fix should fail

(I decided I liked this approach better than pre-deploy hooks.)