subtle variant for nav-tabs on gray background

Not sure about how this should be incorporated, or whether it should go into Sassquatch or have a different name, but here's the deal:

Currently, mobile web tabs are generally on a white background provided by doc-box:
![screen shot 2014-03-11 at 3 18 05 pm](https://f.cloud.github.com/assets/1885153/2389788/e7addf0a-a951-11e3-848e-be10c310aee9.png)

However, the new design for member profile places them neatly outside of the touchList.doc-box:
![screen shot 2014-03-11 at 3 19 14 pm](https://f.cloud.github.com/assets/1885153/2389804/0f75d600-a952-11e3-9df2-c43896832bf7.png)

This `nav-tabs--subtle` variant darkens the tabs in their on and off states. I'm hoping that future designs will also peel the tabs out of the doc-box so we can reduce the number of nested doc-* divs we have to deal with.