WORK IN PROGRESS.

A Rust wrapper for Duktape. Things to do before this is minimally useful:

• Handle non-UTF-8 strings.
• Call JavaScript functions by name.
• Define functions.
  • Call specified Rust functions from JavaScript.
  • Return errors from Rust to JavaScript.
• Convert to use Encodable/Decodable everywhere.
  • Convert parameters to use Encodable.
  • Replace Value with serialize::Json.
  • Convert return values to use Decodable.
• Add nice macros.
  • Provide macro for calling functions.
  • Provide macro for defining functions.