Xkcd: added next button

This is a continuation of #398. I accomplished the hiding of the next button by calling http://xkcd.com/info.0.json/, which will always have the latest comic's info. Then it is just a matter of comparing the numbers in the next url and in the new call's response and deleting the link accordingly. There may have been a better way to do this than string parsing, though, as I'm not really familiar with Handlebars.

I added a new endpoint, like @jagtalon said to do. This means that the test file had to get updated, and as a result is a little more confusing, which may be a problem for the people using it as an example (linked from [here](https://duck.co/duckduckhack/spice_tests))