Add tag support

I thought I'd give tag support a shot. Here we have basic support for tagging articles and the creation of the tag pages using the included template when the site is built. 

Tag pages look like this:

![rom blog 2014-12-31 18-09-38](https://cloud.githubusercontent.com/assets/9002/5591514/37756432-9118-11e4-9404-901b5141d2a7.png)
![rom blog 2014-12-31 18-09-50](https://cloud.githubusercontent.com/assets/9002/5591515/3ec547ca-9118-11e4-8c27-da3a0766d5e6.png)

The tags are displayed in the sidebar when you are viewing an article:

![rom blog rom 0 5 0 released 2014-12-31 18-10-28](https://cloud.githubusercontent.com/assets/9002/5591516/55df3f10-9118-11e4-8b13-53e5d1f0225c.png)

I kinda like that, it keeps the article nice and clean, but give some additional context. This happens via a `content_for(:sidebar_tags)` block. I also added a `content_for(:sidebar)` block for other stuff we may want to shove in there.

And in case you missed it, the sidebar title is changed:

![rom blog 2014-12-31 18-12-32](https://cloud.githubusercontent.com/assets/9002/5591522/a45a9c5c-9118-11e4-8c8d-7a012ae67def.png)

:fireworks: Happy New Year! :fireworks: