Allow to use other annotations or to change behavior of them easily.

Hi,

I would like to use `swagger-php` in my Symfony application to generate documentations.

I have already many annotations to build swagger models and i would like to use them to interact with `swagger-php`.

At this moment, it's impossible to use an other one annotation than predefined annotation or to edit behavior of theses annotations. That's why i purpose to use a `ProcessManager` to implements many `processors` which will create swagger models. By this way, use will be able to interact with any annotation in any context (Class, Method, Property).

For sure, this PR is totally **backward compatible**.

If you have any question/suggestion/doubt ... feel free to ask me.

Regards