Improve user experience and platform detection

• Do not run the incremental recipe if the first build has failed (#174)
• alienv complains if attempting to load a non-existing package (#237)
• alienv prints the just-loaded packages in enter and load (#237)
• Better architecture detection (#222)
• Ship alienv shell helper with the pip distribution (#227)
• Detect latest Ubuntu 16.04 and Debian 7/8 (#245)