Add environment support

@tlunter @bfulton @adamjt 

This pull request adds supports for environments, which are simple a set of deploys. Below is the definition of an environment:

```ruby
deploy :backend do
  ...
end

deploy :frontend do
   ...
end

deploy :load_balancer do
   ...
end

env :staging do
  deploy :backend
  deploy :frontend
  deploy :load_balancer
end
```

When an environment is defined, a multitask is also defined which will perform all of the deploys in that environment in parallel. Execute `bundle exec rake aerosol:env:staging` to run all deploys for the `:staging` environment in this example.

I'm thinking patch version bump for the next release.