Implement "isPermaLink" for item:guid

[neopostmodern]

item:guid has to be a URL unless the isPermaLink attribute is set to false.
I've added this functionality targeting RSS 2.0 and updated tests (for which I need to mangle types a bit -- did these tests pass before I checked them out?)
I believe that the behavior for Atom could/should be updated in some way, because it allows non-URL item:id values (Example), but which wouldn't be allowed in RSS 1.0. But this would cause major infrastructure-changes I guess, wheres just adding the isPermaLink attribute should be non-breaking.

[coveralls]

Coverage increased (+0.6%) to 96.316% when pulling 893e463 on neopostmodern:master into 919ad32 on jpmonette:master.

[dainissilamikelis]

Please add this

[senechaux]

@neopostmodern, I've solved all conflicts in a new PR (#177) to speed up the merge of this feature. I don't want to steal your job or take advantage of it. Please, let me know if you want me to solve conflicts in your master branch so your PR will be ready to merge.

[neopostmodern]

@senechaux Thanks for your effort. I've just merged the master into my repo again – if that is good to go I'd be happy to use this PR; if it isn't I don't mind using yours instead either.
Most of all we need someone who can merge this – @youssmoh, @jpmonette, who could take care of this?

[senechaux]

@youssmoh, @jpmonette, is it possible to merge this PR? it's already been approved.

[marco-msg-ferrari]

Please integrate this 👍

[senechaux]

@youssmoh, @jpmonette, please, let me know if there is something I can help to merge this PR

[antiantivirus]

Please integrate this

[mgoldenberg-nv]

please merge this PR