core.py: Package.branch: support detached HEAD states
core.py: Package.repository: don't ignore exceptions
core.py: Package.branches: handle repository load failure more gracefully
core.py: Package.build(): fix CWD variable reference in `check_call`
util.py: Table.render(): removed debug `print`
dependency.py: added yaml (PyYAML) to PACKAGES
__init__.py: load dependency.py on-demend
README.md: updated to include info on auto-dependency install, etc