Review module structure and how functions are exported to client libraries.

[andrewdavidmackenzie]

As mentioned in https://github.com/andrewdavidmackenzie/libproc-rs/pull/22 I don't really like how the external API looks - but I still need to learn rust best practices here.

So, sorry if ugly in the meantime.

Suggestions for improvements or PRs from people who know how it should be done are very welcome.