Merge pull request #1 from superfeedr/master

Merging upstream changes.
Use rel='canonical' link if present.
Fix missing array key.
Adding categories.
Use alternate link if no canonical link found.

Some feeds don't provide a "canonical" link -- "alternate" is probably
better than a "replies" link.
Further improvement would be to read the individual link href's and
determine which one is most likely to be a good permalink.
Merge pull request #11 from jeanfw/master

Improved link support; added categories as well;