Cleaning
Handle DOMNode and DOMCrawler in Element's filter method. (Fix #36)

There is a BC break in symfony/dom-crawler 2.3.0 :
https://github.com/symfony/DomCrawler/blame/09f0cd5980511ee77c2e8142405ccd5c867506b9/CHANGELOG.md#L8
Revert the isEmpty change
Add an abstraction above DOM crawler and native DOMnodes
Use the new node abstraction in the Element asserter
Add patch method