Improved configuration to be more generic, added the ability to customiz...

...e flash css class, added bootstrap3 compatibility.

| Q             | A
| ------------- | ---
| Bug fix?      | no
| New feature?  | yes
| BC breaks?    | yes
| Deprecations? | no
| Tests pass?   | yes
| Fixed tickets | no
| License       | MIT
| Related PR   | https://github.com/sonata-project/SonataAdminBundle/pull/1874 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