Move Framed from tokio-proto to core

This commit extracts the concrete implementation of `FrameIo` in tokio-proto to
tokio-core under the name `EasyFramed`. This extraction is accompanied with a
new `EasyBuf` buffer type to work with when parsing types.

The purpose of this movement is to provide a clear and easy entry point at the
`FramedIo` layer for those who need it. Eventually these buffer types will get
replaced or moved to the `bytes` crate, but in the interest of an 0.1 release
and remaining backwards compatible with the tokio-core 0.1 release this is
adding a separate module.

src/io/mod.rs

@@ -131,6 +131,9 @@ pub trait Io: io::Read + io::Write {
 /// value that indicates "would block" the current future's task is arranged to
 /// receive a notification when the method would otherwise not indicate that it
 /// would block.
+///
+/// For a sample implementation of `FramedIo` you can take a look at the
+/// `EasyFramed` type in the `easy` module of htis crate.
 pub trait FramedIo {
     /// Messages written
     type In;