implement mix package and related tasks re #1124
changes from PR discussion

1: ensure_loaded?

3: ebin

2: assume path/ebin to be packaged

6: accept project path & assume ebin

8: include priv in archive

5: local.uninstall handles packages too

7: local.install [] will install a package

7: mix new .gitignore *.ez files
local.uninstall warn on uninstall package task
Merge pull request #1223 from jeremyjh/1124-mix-local-arch

add mix package and related tasks