Use rustc-serialize instead of serialize crate

[cyderize]

The serialize crate is being deprecated in favour of the one from
crates.io. PR: rust-lang/rust#19820.

Also, I changed the deprecated into_string() call to to_string(), although maybe I should be using to_owned() from std::borrow::ToOwned?

[chris-morgan]

I'm much more inclined to just remove that part; there's really no reason
to add that dependency that I can see. The purpose of libserialize has been
clarified (it's for rustc) and mucell doesn't fit into that.

(I haven't taken action on this yet as I am on a very dodgy and slow
Internet connection for a few days; I'll probably take action early next
week.)

[cyderize]

That makes sense. I've created a PR #7 that removes the libserialize dependency, if that's useful to you.