Big rewrite and version 0.2

Version 0.2!

A complete rewrite that fixes, among other things, my disastrous misunderstandings about how `dotenv` works.

Everything, up to the public API, has been completely revamped.

I'd expect this API to be more stable, although it's hard to say so when the language itself keeps changing so fast! I hope to bring a stable release shortly after Rust hits v1.0.0, although there are many things missing for me to feel confident doing so.