Added support for KnpMenu v2-alpha

| Q             | A
| ------------- | ---
| Bug fix?      | no
| New feature?  | yes
| BC breaks?    | yes
| Deprecations? | no
| Tests pass?   | yes
| Fixed tickets | #1712, https://github.com/phiamo/MopaBootstrapBundle/issues/606
| License       | MIT
| Related PR        | 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/SonataBlockBundle/pull/107 https://github.com/sonata-project/sandbox/pull/129 https://github.com/sonata-project/SonataPageBundle/pull/250 https://github.com/sonata-project/SonataNewsBundle/pull/149

*WARNING* This PR makes the AdminBundle unstable as it depends on an unstable KnpMenu version. However, it should be pretty stable regarding features.

This comes for giving an answer to issues https://github.com/sonata-project/SonataAdminBundle/issues/1712 and https://github.com/phiamo/MopaBootstrapBundle/issues/606 ; this will allow full support of MopaBootstrapBundle.