Filesystem manipulation APIs. (#323)

This patch adds a new crate: tokio-fs. This crate provides a wrapper
around `std` functionality that can only be performed using blocking
operations. This primarily includes filesystem operations, but it also
includes standard input, output, and error access as these streams
cannot be safely switched to non-blocking mode in a portable way.

These wrappers call the `std` functions from within a `blocking`
annotation which allows the runtime to compensate for the fact that the
thread will potentially remain blocked in a system call.

src/fs.rs

@@ -0,0 +1,13 @@
+//! Asynchronous filesystem manipulation operations.
+//!
+//! This module contains basic methods and types for manipulating the contents
+//! of the local filesystem from within the context of the Tokio runtime.
+//!
+//! Unlike *most* other Tokio APIs, the filesystem APIs **must** be used from
+//! the context of the Tokio runtime as they require Tokio specific features to
+//! function.
+
+pub use tokio_fs::{
+    file,
+    File,
+};