Remove trailing spaces
Abstract out builder from stemcell
space around operations
Fix wrong debug log message
Make BaseBuilder an abstract class

Subclass has to implement package_stemcell. Subclass generates a list of
files, then package them into a tgz file.
Refactor
Multi-line block uses do..end
Should chdir to prefix to untar
Fix wrong banner
Fix command info
Code alignment
Merge pull request #1 from anfernee/rhel

Some refactors