Add <link rel="next"> (and "prev") tags.

These are handy for programmatic tools indexing the comic. Like, say, Comic Rocket :-) or Google (http://googlewebmastercentral.blogspot.com/2011/09/pagination-with-relnext-and-relprev.html).
Merge pull request #12 from jameysharp/patch-2

Add <link rel="next"> (and "prev") tags.

I'm merging this anyway; there are some planned changes for templates which should incorporate this fix. Thanks!