Added new spool type "service" to allow custom spool implementations
merged branch snc/spool-service (PR #50)

This PR was squashed before being merged into the master branch (closes #50).

Discussion
----------

Added new spool type "service" to allow custom spool implementations

This PR makes it possible to use custom spool implementations (see #48 and #49).

```
swiftmailer:
    spool:
        type: service
        id: swiftmailer.spool.redis
```

If the implementation is OK for you I will update the tests ;-)

Commits
-------

2a6d7a5 Added new spool type "service" to allow custom spool implementations