add fixed sidebar

Fixes #31 (at least partially)

# Medium size
![screen shot 2014-01-29 at 5 03 57 pm](https://f.cloud.github.com/assets/150352/2034824/4878dccc-8931-11e3-9c98-b79f13dd8437.png)

# Smaller
![screen shot 2014-01-29 at 5 04 26 pm](https://f.cloud.github.com/assets/150352/2034828/580548b0-8931-11e3-936d-d7845a24fb02.png)

# Smaller yet (mobile?)
All position:fixed are removed
![screen shot 2014-01-29 at 5 05 03 pm](https://f.cloud.github.com/assets/150352/2034833/6f83f824-8931-11e3-8709-89abce6658e5.png)
![screen shot 2014-01-29 at 5 06 12 pm](https://f.cloud.github.com/assets/150352/2034842/97a3cf1e-8931-11e3-8295-341b128f03a8.png)