[DomCrawler] Added auto-discovery and explicit registration of namespaces in filter() and filterByXPath()

| Q | A 
| --- | ---
|Bug fix: | no
|Feature addition: |yes
|Backwards compatibility break: | yes, default namespace is no longer removed in the `addContent` method
|Symfony2 tests pass: | yes|
|Fixes the following tickets: | #4845
|Todo: | -
|License of the code:| MIT
|Documentation PR: | symfony/symfony-docs#2979

* added support for automatic discovery and explicit registration of document namespaces for `Crawler::filterXPath()` and `Crawler::filter()`
* improved content type guessing in `Crawler::addContent()`
* [BC BREAK] `Crawler::addXmlContent()` no longer removes the default document namespace

I mentioned in #4845 it would probably be possible to use [DOMNode::lookupNamespaceURI()](http://www.php.net/manual/en/domnode.lookupnamespaceuri.php) to find a namespace URI by given prefix. Unfortunately we cannot use it here since we'd have to call it on a node in the namespace we're looking for.

Current implementation makes the following query to find a namespace:
```php
$domxpath->query('(//namespace::*[name()="media"])[last()]')
```