new article navigation that uses tags to highlight current local nav

Make local nav highlighting more relevant by taking article tags into consideration.
This change is hidden behind 'nav-highlight' http switch until it's fully validated.

Before - wrong local nav section highlighted 
![png](https://cloud.githubusercontent.com/assets/1492162/3510849/6f03f15a-06aa-11e4-98de-0b86bbc65d32.png)

Article tags
![png-2](https://cloud.githubusercontent.com/assets/1492162/3510852/730ac6de-06aa-11e4-987d-ebd96d7cda68.png)

After - closest match to current context
![png-1](https://cloud.githubusercontent.com/assets/1492162/3510874/9b043738-06aa-11e4-805c-edc53fa34ab4.png)