add mix package and related tasks

This is related to #1124. This adds three related tasks and supporting modules:

mix package -- package up any project into an erlang archive format file
mix local.install.package -- install current project into MIX_HOME/tasks
mix local.uninstall.package -- uninstall specified package (app) name

This also adds the logic within Mix.Task and Mix.Local to find the archive files in MIX_HOME/tasks and add (their inner code path) to the code paths.