Doctrine integration

I haven't found any previous discussions about this topic, so I start one here:

One thing that is missing from this gem (and we had it in capifony) is doctrine integration.

I have this basic list of doctrine tasks:

``` ruby
namespace :doctrine do
namespace :schema do
desc "Drop doctrine schema"
task :drop do
on roles(:app) do
invoke "symfony:console", "doctrine:schema:drop", "--force"
end
end

desc "Create doctrine schema"
task :create do
on roles(:app) do
invoke "symfony:console", "doctrine:schema:create"
end
end
end

namespace :migrations do
desc "Execute doctrine migrations"
task :migrate do
on roles(:app) do
invoke "symfony:console", "doctrine:migrations:migrate", "--no-interaction"
end
end
end

namespace :fixtures do
desc "Load doctrine fixtures"
task :load do
on roles(:app) do
invoke "symfony:console", "doctrine:fixtures:load", "--no-interaction"
end
end
end
end
```

I am aware of https://github.com/glooby/capistrano-symfony-doctrine, but I think it would be better to have it in the symfony package. Also, I propose the following improvements:
- Support different entity managers
- Roles and environment should be defined by the user when using the specific tasks in the configuration

What do you think?

Review the issue's proposed Doctrine schema, migration, and fixture tasks, along with the referenced capistrano-symfony-doctrine integration. Clarify support for different entity managers and user-defined roles and environments; done would require an agreed integration design and implementation scope.

Written by the indexing model from the issue text.