Use old *_build folder in case of a new branch
Fix error if .sconsign.branch is missing. In this case, the current
*_build folder is re-used.
fixed scoping of some re-used variables
create branch folder in cache even when .sconsign.brach does not exists
removed if that always evaluates to true
Merge pull request #34 from daschuer/virtualize

Use old *_build folder in case of a new branch