EZP-22210: Implement legacy bundles

> Status: in progress
> **Story:** http://jira.ez.no/browse/EZP-22210
> **Specifications:** [doc/specifications/legacy_bundles/legacy_bundles.md](https://github.com/ezsystems/ezpublish-kernel/blob/EZP-22210-legacy_bundles/doc/specifications/legacy_bundles/legacy_bundles.md)
> **ezsystems/ezpublish-legacy PR:** https://github.com/ezsystems/ezpublish-legacy/pull/880
> **ezsystems/ezpublish-community PR:** https://github.com/ezsystems/ezpublish-community/pull/109
> **ezsystems/comments-bundle PR:** https://github.com/ezsystems/CommentsBundle/pull/6

### Summary
A Legacy Bundle is a Symfony2 bundle that contains eZ Publish legacy extensions.
Those are automatically injected into legacy  as soon as the bundle is enabled.

See the [specifications](https://github.com/ezsystems/ezpublish-kernel/blob/EZP-22210-legacy_bundles/doc/specifications/legacy_bundles/legacy_bundles.md) for more details.

### Example
![Structure of a legacy bundle](https://f.cloud.github.com/assets/235928/2169772/7cbbc09a-9560-11e3-849e-e345fe507d31.PNG)

See the pull request to the comments bundle: https://github.com/ezsystems/CommentsBundle/tree/EZP-22210-legacy_bundles/ezpublish_legacy/ezcommentsbundle.

### Tasks
- [x] Update specifications in regards to the interface
- [x] Make the install script smarter (don't remove target if it exists and isn't a link to the existing folder)
- [x] Merge https://github.com/ezsystems/ezpublish-legacy/pull/880
- [ ] Merge https://github.com/ezsystems/ezpublish-community/pull/109
- [ ] Merge https://github.com/ezsystems/CommentsBundle/pull/6
- [ ] **before merging**, change back the required version of comments-bundle