[WIP] Bootstrap3 front theming

See https://github.com/sonata-project/SonataAdminBundle/issues/1723

![capture decran 2014-03-26 a 16 17 09](https://f.cloud.github.com/assets/1321977/2526642/75d65dec-b4fc-11e3-9f11-30f3d3ccf689.png)
![capture decran 2014-03-26 a 16 18 17](https://f.cloud.github.com/assets/1321977/2526643/75da8390-b4fc-11e3-9e87-ce45ae31f01e.png)
![capture decran 2014-03-26 a 16 18 39](https://f.cloud.github.com/assets/1321977/2526644/75e05158-b4fc-11e3-8ecd-a77b5e61a5ef.png)

Left to do:
- [x] Find a way to manage resources (bootstrap, sb-admin, x-editable, select2, ...)
- [x] Check that there are no regressions
- [x] Document BC Breaks in UPGRADE.md
- [ ] Create new branches for SonataUserBundle, SonataMarkItUpBundle, SonataDoctrineORMAdminBundle, SonataNewsBundle, SonataTimelineBundle, SonataMediaBundle, SonatajQueryBundle, SonataPageBundle and the demo
- [ ] Merge https://github.com/sonata-project/SonataUserBundle/issues/381
- [ ] Merge https://github.com/sonata-project/SonataMarkItUpBundle/pull/9
- [ ] Merge https://github.com/sonata-project/SonataDoctrineORMAdminBundle/pull/333
- [ ] Merge https://github.com/sonata-project/SonataNewsBundle/pull/187
- [ ] Merge https://github.com/sonata-project/SonataTimelineBundle/pull/17
- [ ] Merge https://github.com/sonata-project/ecommerce/pull/206
- [x] Merge https://github.com/sonata-project/SonatajQueryBundle/pull/18
- [ ] Merge https://github.com/sonata-project/sandbox/pull/341
- [ ] Merge https://github.com/sonata-project/SonataMediaBundle/pull/521
- [ ] Merge https://github.com/sonata-project/SonataPageBundle/pull/312
- [ ] Update composer.json to point on the new branches