Initial commit of feed package.

Moving some parsing logic around and starting to think about how I want
to make this thing extensible.

Cleaning up some things.

Adding some things for testing.

Updating @since tags and general cleanup.

Cleaning up unit tests.
Tests for JFeed
Fleshing out a lot of unit tests.
Adding tests for JFeedParser
Tests for remaining feed code.
Finishing up the RSS parser implementation.
Update the feed package for deprecation of `JXMLElement`.
Move feed package tests to the new unit test structure.
General cleanup.
Merge pull request #1622 from LouisLandry/feed

New Feed Package.