package_env improvements

[jowrjowr]

The package_env feature takes some figuring out as it doesn't work fully out-of-the-box compared to the other resources this module provides.

I'd like to suggest some usability improvements:

Easy:

1. Make it clear in the documentation that you have to generate the /etc/portage/env and manage the environment files yourself, and link http://wiki.gentoo.org/wiki//etc/portage/env for clarity.

Harder:

1. Have the module build the env directory itself
2. Build a new provider/type specifically for environment management.

Just doing the easy step would have saved me a lot of time because I personally overestimated the module's capabilities (which have been otherwise fantastic).