Highlight the section we're in, in the navigation. Fixes #483.

Looks like this:

![image](https://cloud.githubusercontent.com/assets/12306/2869228/2e2dba9c-d25e-11e3-9010-f3ab36143589.png)

The request in #483 raises some problems with the way we structure our navigation right now. 

If I click on "All Checks", then click on a link to an entity, I lose context about where I am in the navigation. 

It's likely there's duplication in the navigation, and we should streamline it to look something like this:

![image](https://cloud.githubusercontent.com/assets/12306/2869222/ef507242-d25d-11e3-9eba-d8ef5fadd152.png)

Entities + checks become dropdowns where you can select all/failing/in maintenance/etc.