Provide optional features on tokio crate (#808)

Disabling all features means the only dependency is `futures`.

Relevant pieces of the API can then be enabled with the following features:

- `codec`
- `fs`
- `io`
- `reactor`
- `tcp`
- `timer`
- `udp`
- `uds`

This also introduces the beginnings of enabling only certain pieces of the `Runtime`. As a start, the entire default runtime API is enabled via the `rt-full` feature.

src/io.rs

@@ -51,6 +51,7 @@ pub use tokio_io::{
 };
 
 // standard input, output, and error
+#[cfg(feature = "fs")]
 pub use tokio_fs::{
     stdin,
     Stdin,