Adds a new constructor and init functions for custom env vars.

[jimmycuadra]

Three new methods/functions that are like existing ones, but allow you to specify the name of the environment variable to parse:

• Logger::from_env, which is like Logger::new.
• init_from_env, which is like init.
• try_init_from_env, which is like try_init.

I also changed the docs for the init functions slightly to make it clearer what the difference is between them all when reading the docs for the crate root.

I haven't added any new tests, but I'd be happy to if desired. Let me know if there is a preferred way of working with environment variables to keep tests deterministic.

Refs #23
Refs rust-lang-nursery/log#83

[KodrAus]

Thanks again @jimmycuadra!

I think this looks good. There probably isn't a whole lot of value testing the environment vars, since they just hand off to parse where the real work happens.