add ability to tag posts (Fixes GH-24)

tags! :tada:  i'm super open to feedback on how tags should be done, this is just my thinking of it. tags can be added to posts, and posts can be filtered by tag by clicking on the tag. fixes GH-24. screenshots:

![screen shot 2014-12-28 at 4 22 28 pm](https://cloud.githubusercontent.com/assets/1459603/5565395/ff82466a-8ead-11e4-9106-b643ade6fe3c.png)
![screen shot 2014-12-28 at 4 22 32 pm](https://cloud.githubusercontent.com/assets/1459603/5565396/ff95f46c-8ead-11e4-9fc0-f6bd50eda73f.png)
![screen shot 2014-12-28 at 4 22 48 pm](https://cloud.githubusercontent.com/assets/1459603/5565394/fbb6e4aa-8ead-11e4-8707-ebde31866adf.png)