Various sidebar, tab aesthetic tweaks

They're all fairly minor tweaks. Minor increases in vertical spacing with sidebar row padding and tab height, simplifies tabs by hiding the close icon (#66), and selected sidebar rows are a lot more subtle.

![soda](https://f.cloud.github.com/assets/322127/206715/08c0f55c-81d9-11e2-9ebc-6e2fa3cade28.png)