tzinfo: claify tz objects in Docs and add timezone parsing on arrow.get()

This fulfils [issue #58](https://github.com/crsmithdev/arrow/issues/58).

In some places you use the `tz` kwarg for objects that may be a string or may be a `tzinfo` object, but it seems a bit unclear if that is the intention, or if it's to avoid collision with the imported `tzinfo` object when an `isinstance` check is raised. I figured for a first crack at it, I'd add the feature without any breaking changes.