Unify local and remote deps, add specs

I think it'd be great if we could avoid separate `local_dep` and `remote_dep`, when they're so similar. This pull request adds a new syntax for meeting deps on the guest:

```ruby
babushka.meet "a local dep"
babushka.meet "a local dep", :params => {:key => "value"}
babushka.meet "a remote dep", :source => "the owner", :params => {:key => "value"}
```

This is a lot more idiomatic, and avoids confusion around the different syntax (and different numbers of parameters for each). The old methods are noted as deprecated, and a warning message is logged if they are used (however, they still work perfectly fine for now, the same way as before).

This pull request also adds a Vagrantfile (for testing during development), as well as a small suite of RSpec tests.