Adding node name getter with tests

This PR was submitted on the symfony/DomCrawler read-only repository and moved automatically to the main Symfony repository (closes symfony/DomCrawler#17).

Based on the use case came up in a SO question (http://stackoverflow.com/questions/22564465/getting-the-tag-name-of-a-element-crawled-with-domcrawler-in-php) I propose a new helper function to get the name of the active node