New $composer_home fact and exec environment

In Puppet versions > 3.0, environment variables such as `$HOME` no longer exist. Composer relies on `$HOME` to store metadata and fails to run in Puppet versions after 3.0. This PR adds a simple `composer_home` fact which returns the current users home folder and then uses that as the default for the `$COMPOSER_HOME` environment variables, which composer will use in lieu of `$HOME`.