[WIP] 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        | ~

- [ ] Give some love to the UI (need your feedbacks)
- [ ] Write tests
- [ ] 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`:
![sans titre](https://cloud.githubusercontent.com/assets/667519/5273692/deb7a772-7a8a-11e4-8cee-257e8948da50.png)

Here is the output for `security:encode-password`:
![capture d ecran 2014-12-03 a 01 11 14](https://cloud.githubusercontent.com/assets/667519/5273707/09ee53fa-7a8b-11e4-80e6-2274ba13018a.png)



IMHO, you should catch exceptions to display nice errors (ie: is the user class valid?)