Fix Atom/RSS feed parsing due to changes in jteeuwen/go-pkg-xmlx.

Due to recent changes involving how values are dealt with in xmlx, update
the RSS/Atom parsing.  Instead of using the Value property of an xmlx Node,
use the new GetValue function offered in PR jteeuwen/go-pkg-xmlx#15.
Fix RSS feed parsing due to changes in go-pkg-xmlx.

Due to the PR jteeuwen/go-pkg-xmlx#16, when using SelectNodes there is no
longer any hidden recursion.  Due to RSS's structure, there is a root document
node.  These two pieces break the RSS parsing.  Fix by first selecting the
root document node, and then selecting the channels for parsing.
Merge pull request #23 from MJDSys/FixFeedParsingForxmlx

Fix Atom/RSS feed parsing due to changes in jteeuwen/go-pkg-xmlx.