Add @alfredxing to the @jekyll/core team.

Team post because he's awesome! :tada: 

![screen shot 2014-12-17 at 11 31 00 am](https://cloud.githubusercontent.com/assets/237985/5477432/35b8f746-85e0-11e4-9c57-3532fc612b2c.png)
For sure, a lot of the framework and ideas, and quite a few of the tests were built by @mattr-; he deserves a lot more credit on that feature.