Restructure and trim down the library

This commit is a significant overhaul of the library in an effort to head
towards a stable API. The rope implementation as well as a number of buffer
implementations have been removed from the library and will live at
https://github.com/carllerche/bytes-more while they incubate.

**Bytes / BytesMut**

`Bytes` is now an atomic ref counted byte slice. As it is contigous, it offers
a richer API than before.

`BytesMut` is a mutable variant. It is safe by ensuring that it is the only
handle to a given byte slice.

**AppendBuf -> ByteBuf**

`AppendBuf` has been replaced by `ByteBuf`. The API is not identical, but is
close enough to be considered a suitable replacement.

**Removed types**

The following types have been removed in favor of living in bytes-more

* RingBuf
* BlockBuf
* `Bytes` as a rope implementation
* ReadExt
* WriteExt

tests/test_mut_buf.rs

@@ -0,0 +1,47 @@
+extern crate bytes;
+extern crate byteorder;
+
+use bytes::BufMut;
+use std::usize;
+
+#[test]
+fn test_vec_as_mut_buf() {
+    let mut buf = Vec::with_capacity(64);
+
+    assert_eq!(buf.remaining_mut(), usize::MAX);
+
+    unsafe {
+        assert!(buf.bytes_mut().len() >= 64);
+    }
+
+    buf.copy_from(&b"zomg"[..]);
+
+    assert_eq!(&buf, b"zomg");
+
+    assert_eq!(buf.remaining_mut(), usize::MAX - 4);
+    assert_eq!(buf.capacity(), 64);
+
+    for _ in 0..16 {
+        buf.copy_from(&b"zomg"[..]);
+    }
+
+    assert_eq!(buf.len(), 68);
+}
+
+#[test]
+fn test_put_u8() {
+    let mut buf = Vec::with_capacity(8);
+    buf.put_u8(33);
+    assert_eq!(b"\x21", &buf[..]);
+}
+
+#[test]
+fn test_put_u16() {
+    let mut buf = Vec::with_capacity(8);
+    buf.put_u16::<byteorder::BigEndian>(8532);
+    assert_eq!(b"\x21\x54", &buf[..]);
+
+    buf.clear();
+    buf.put_u16::<byteorder::LittleEndian>(8532);
+    assert_eq!(b"\x54\x21", &buf[..]);
+}