Add Berkshelf support to Knife Solo

This will add:

* Berkshelf support to `knife solo cook`
* A generated Berksfile on `knife solo init`
* A flag to skip Berkshelf file generation in `knife solo init`

It is important to note that Knife Solo currently does not require Ruby 1.9 but Berkshelf does. You may want to add the following line to your gemspec.

```ruby
Gem::Specification.new do |s|
  ...
  s.required_ruby_version = ">= 1.9.1"
end
```

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