This commit completely rewrites this crate from the ground up,
supporting serde at the lowest levels as I believe serde support was
intended to do. This is a major change from the previous versions of
this crate, with a summary of changes being:

* Serialization directly to TOML is now supported without going through
  a `Value` first.

* Deserialization directly from TOML is now supported without going
  through a `Value`. Note that due to the TOML format some values still
  are buffered in intermediate memory, but overall this should be at a
  minimum now.

* The API of `Value` was overhauled to match the API of
  `serde_json::Value`. The changes here were to:

  * Add `is_*` accessors
  * Add `get` and `get_mut` for one-field lookups.
  * Implement panicking lookups through `Index`

  The old `index` methods are now gone in favor of `get` and `Index`
  implementations.

* A `Datetime` type has been added to represent a TOML datetime in a
  first-class fashion. Currently this type provides no accessors other
  than a `Display` implementation, but the idea is that this will grow
  support over time for decomposing the date.

* Support for the `rustc-serialize` crate has been dropped, that'll stay
  on the 0.2 and 0.1 release trains.

* This crate no longer supports the detection of unused fields, for that though
  you can use the `serde_ignored` crate on crates.io