Add custom.css.sass where bootstrap is imported

This has something to do with the assetpipeline, bootstrap
can be imported just after the sprocket directive. Right now
problem is that variables can be used across sass files. Still
need to be check the right way.

http://stackoverflow.com/questions/8887824/persisting-scss-variables-in-rails-asset-pipeline
http://www.railstutorial.org/book/filling_in_the_layout#code-bootstrap_sass

Signed-off-by: Oscar Sanhueza <oscarsanhueza@gmail.com>