Revert "Use custom RSS encoding"

This reverts commit 0afbf1d06c230c4c21696d5c102d5f238077b93d.

After some discussion on snap_framework, it turns out that RSS readers should
understand html encoded html in the <description/> tag. Thus, there's no
reason to use a custom RSS generator and a CDATA wrapper -- RSS readers should
Just Work.

Closes #1