As a follow up to issue #699 update SimplePie from 1.2.1-dev (around
6years old) to current git master (1.4.4-dev). We use the master branch
until the next release because it contains our patch for enclosure
titles already. The other patches in the library from us can be done by
configuring the SimplePie object in our code instead.
Used composer to manage this library and use class autoloading.
Add some unit tests for include/feedutils.php, but the interesting parts
are unfortunately not testable with the current code.