PEP8 and edit cleanup
FEA: Warn that updating EPD can cause downgrades
FEA: check for incompatible actions and options
BUG: don't terminate install loop when no update needed of a package

Bug introduced in PR # 50, 6a1d6ef0, auto-prompting for credentials
Merge branch 'master' into update-all_switch
Merge pull request #85 from enthought/update-all_switch

Cleanup enpkg command-line handling