Revert "remove content length header because we cant make sure that this is the only output. For instance in the admin page you need to render a subset of the page as a template, setting a content length header breaks the page cc @tanghus"

This reverts commit 09eefe1d757189d968d23ab7399419bb1231ecec.
Add App:part() method which doesn't send any headers
Remove test for actual output.
Be away, unicorns.
Merge pull request #36 from owncloud/render_parts

Add App::parts method which doesn't send any headers.