Feature/add option for gray selected sidebar

Related #81

Add optional setting `  "flatland_gray_selected_sidebar"` 

In Default, the selected sidebar color looks like 

![2014-03-13 20 04 00](https://f.cloud.github.com/assets/1506738/2408623/3742cc4e-aa9f-11e3-9d6c-4eb0b6612676.png)

If you set  `"flatland_gray_selected_sidebar"` to  `true`,  the selected sidebar looks like 

![2014-03-13 20 05 06](https://f.cloud.github.com/assets/1506738/2408632/5c215288-aa9f-11e3-8d8b-bbf6e877126d.png)