Update library to match the current nightly

Hi! I've made a few changes to make rust-csv compilable under the current rust-nightly, and to solve a few warnings.

It was mostly about the `~"foo"` removal, and the fact that `~[T]` isn't growable anymore. I also removed a couple functions that were dead code, and some unnecessary uses of `~` (which is currently being discouraged unless necessary).

If you feel this is too big, or that it touches on too many different things, I can split it up