rss didn't work for me, this fixes it

When I initially tried to get rss working, I was getting a couple of weird errors.

1 - xmlpaths wasn't working
2 - an object was not being found, namely RSS

You can see in the diff I changed xmlTreeParse to use internal nodes, this seems to fix 1.

I renamed `rss` to `main.rss`, this fixes 2.

This has enabled me to generate both the main feed (http://rmflight.github.io/rss.xml), and R specific feed for my site (http://rmflight.github.io/tags/R.xml), so I'm pretty sure this is the way it should be.