environment variables for emerge

Hi,

I added the ability to set environment variables before emerge, eg:
```
    package { "mongodb":
        ensure => "2.2.2-r1",
        environment => "EPYTHON=python2.7",
        category => "dev-db",
        keywords => "~amd64",
    }
```
(since mongodb won't compile with python 3.x)
pull request since I think it might be interesting to other people too eg:
http://projects.puppetlabs.com/issues/6400
although not Gentoo... but still...