Ensure text within XML elements is properly escaped

[misfo]

I tried to subscribe to the The Phoenix blog with Blogtrottr, but it couldn't subscribe because of an XML parsing error (see the parse error here). It looks like an unescaped & was causing the parse error. This PR has xmerl handle the escaping for us.

Fixes #9

Btw, the second commit isn't strictly necessary but it might help prevent invalid XML in the future...

Thanks!

[Betree]

Sounds critical to me as user's content may end up in the feed (ex: a comments feed). Nice work @misfo

Any chance to see this PR merged soon ?