SONATA-195 - Added bootstrap3 support

| Q             | A
| ------------- | ---
| Bug fix?      | no
| New feature?  | yes
| BC breaks?    | no
| Deprecations? | no
| Tests pass?   | yes
| Fixed tickets | no
| License       | MIT
| Related PRs    | https://github.com/sonata-project/SonataAdminBundle/pull/1874 https://github.com/sonata-project/SonataCoreBundle/pull/24 https://github.com/sonata-project/ecommerce/pull/45 https://github.com/sonata-project/SonataUserBundle/pull/305 https://github.com/sonata-project/sandbox/pull/129 https://github.com/sonata-project/SonataPageBundle/pull/250 https://github.com/sonata-project/SonataNewsBundle/pull/149

All related PRs must be merged together