fix README.md

 - pbundle run has been renamed to pbundle exec
 - os.system isn't actually used anywhere
fix typo in pbundler/bundle.py

 - "pgk" instead of "pkg" when building Exception message
remove unused import

 - pointed out by pyflakes
Merge pull request #24 from natano/fix_readme

fix README.md
Merge pull request #25 from natano/fix_pgk_typo

fix typo in pbundler/bundle.py
Merge pull request #26 from natano/remove_unused_imports

remove unused imports
Fix pbundle-py entrypoint

Fixes: #27
Reported-By: Martin Natano @natano