- Support headers in fetch()
- Adds many async fs functions:
  deno.rename() deno.remove(), deno.removeAll(), deno.removeSync(),
  deno.removeAllSync(), deno.mkdir(), deno.stat(), deno.lstat()
  deno.readFile() and deno.writeFile().
- Add mode in FileInfo
- Access error codes via error.kind
- Check --allow-net permissions when using fetch()
- Add deno --deps for listing deps of a script.

and removeSync(), removeAllSync().

As an example of how to implement ops that have both sync and async
versions.

And send() -> sendSync()

throwResolutionError was swallowing unrelated errors.

Originally we planned to have a JS class for each error code. But it
seems better to just have a single DenoError class with a "kind"
property. One nice thing about using an enum instead of classes for
errors is that switch() can be used during error handling instead of a
bunch of instanceof branches.

Refactors handlers.rs

The idea is that all Deno "ops" (aka bindings) should map onto
a Rust Future. By setting the "sync" flag in the Base message
users can determine if the future is executed immediately or put
on the event loop.

In the case of async futures, a promise is automatically created.
Errors are automatically forwarded and raised.

TODO:

- The file system ops in src/handler.rs are not using the thread pool
  yet. This will be done in the future using tokio_threadpool::blocking.
  That is, if you try to call them asynchronously, you will get a promise
  and it will act asynchronous, but currently it will be blocking.
- Handlers in src/handler.rs returned boxed futures. This was to make
  it easy while developing. We should try to remove this allocation.

* Add forgotten --eol argument to 'git ls-files'. Without it, in certain
  edge cases, files might have their mtime restored when it shouldn't.
* Don't parse ls-files output into fields; it's unnecessary.
* Disable the pip cache dir, and stop saving it to the appveyor cache.
* Leverage the 'git -C «dir»' flag to make a script shorter.
* Fix some factually and/or grammatically incorrect comments.