[2.7][SecurityBundle] Added a command to encode a password

| Q             | A
| ------------- | ---
| Bug fix?      | no
| New feature?  | yes
| BC breaks?    | no
| Deprecations? | no
| Tests pass?   | yes
| Fixed tickets | #11206
| License       | MIT
| Doc PR        | ~

- [x] Give some love to the UI (need your feedbacks)
- [x] Write tests
- [x] Write documentation

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The encoder got depend directly from the configuration of the user class in the security.yml.
So the user choose the user type, and, using the method `getEncoder($user)` of the `security.encoder_factory` service, I get the right encoder configured.

*Here is the output for `security:encode-password --show-password`*:
![capture d ecran 2014-12-21 a 15 58 54](https://cloud.githubusercontent.com/assets/667519/5518510/f5c28e8c-892b-11e4-8e42-e51f0dee9138.png)

*Here is the output for `security:encode-password`*:
![capture d ecran 2014-12-21 a 15 58 25](https://cloud.githubusercontent.com/assets/667519/5518509/f1a2698a-892b-11e4-9dba-f94da62ce0fd.png)





Let's remove this.