Upload assets to S3
Upload only styles and scripts
Fix procfile to rerun app when lib/**/* changes
Allow to set "custom_branch" for assets

When custom-branch param is set, travis-web will serve assets from S3
from given branch (the compiled assets are uploaded to S3 on each
build). Cookie will also be set to serve assets from this branch until
disabled.

Custom branch may be disabled by passing disable-custom-branch=true
param.