Enum Namespacing

BREAKING CHANGE: This change will require users to reference
mime types using their namespace. i.e. MediaType::Json.
Rust enums were updated in the latest nightly (rust-lang/rust#18973).

I added pub use in places where the enums wouldn't be poluting user
namespaces with a large amount of names.

I also switched to the upstreams of rust-http and rust-url. It seems that the nickel-org forks no longer diverge from their upstreams. I'm happy to split this change into a second PR.