[2.7][FrameworkBundle] Added a command to lint XML files.

| Q             | A
| ------------- | ---
| Bug fix?      | no
| New feature?  | yes
| BC breaks?    | no
| Deprecations? | no
| Tests pass?   | yes
| Fixed tickets | ~
| License       | MIT
| Doc PR        | ~
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Output:

![capture d ecran 2014-12-28 a 17 58 29](https://cloud.githubusercontent.com/assets/667519/5564380/cc2c174c-8ebb-11e4-809a-47c5e728c557.png)

![capture d ecran 2014-12-28 a 18 00 13](https://cloud.githubusercontent.com/assets/667519/5564379/ca684cc8-8ebb-11e4-9c9c-d9db75949b19.png)

![capture d ecran 2014-12-28 a 18 04 41](https://cloud.githubusercontent.com/assets/667519/5564383/46fb5906-8ebc-11e4-80d4-c5e2b244491a.png)

![capture d ecran 2014-12-28 a 18 05 34](https://cloud.githubusercontent.com/assets/667519/5564384/4e195030-8ebc-11e4-8596-d28ffc41a45e.png)


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- [ ] Write tests.
You should write `null !== $xsd`