[Config] Create XML Reference Dumper

| Q             | A
| ------------- | ---
| Bug fix?      | no
| New feature?  | yes
| BC breaks?    | no
| Deprecations? | yes
| Tests pass?   | no
| Fixed tickets | -
| License       | MIT
| Doc PR        | -

Only Yaml was supported. This PR adds support for XML. This makes it easier to test XML schema's (see symfony-cmf/MenuBundle#114 ), helps us at the docs with our configuration reference and helps others using XML with symfony.

## Todo

 - [x] Prototyped arrays don't work properly
 - [x] Add comments (see Yaml dumper)
 - [x] Add namespaces support

## Side effects

I've moved the reference dumpers to their own namespace and renamed the original reference dumper to `YamlReferenceDumper`. The old one is kept for BC, but deprecated.

/cc @dantleech