New header image
Removed _site to avoid conflicts

Github.com no longer parses the _site anyways since we rely on
custom _plugins that are not executed in the github.com render
process. To mitigate this I manually sync this branch with the
gh-pages branch in the github.com/dashorst/wicket-site repo.
Ignored _site to avoid conflicts

As _site is no longer generated by github it is a liability when
collaborating on the site (e.g. atom.xml constantly gives merge
conflicts). Added _site to .gitignore to remove this constant
pain.