[FrameworkBundle] Added a translation:debug command

| Q             | A
| ------------- | ---
| Bug fix?      | no
| New feature?  | yes
| BC breaks?    | no
| Deprecations? | no
| Tests pass?   | yes
| Fixed tickets | 
| License       | MIT
| Doc PR        | 

This pull request adds a `translation:debug` command that can be used to inspect unused / missing messages and compare them with the fallback ones (if any). This is inspired by this ruby project https://github.com/glebm/i18n-tasks

Example of output:

![demo](https://f.cloud.github.com/assets/1586668/1950894/275ccb72-815f-11e3-96ba-ad393905eac2.png)