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`AppendBuf` has been replaced by `ByteBuf`.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_buf.rs

@@ -0,0 +1,58 @@
+extern crate bytes;
+extern crate byteorder;
+
+use bytes::{Buf, Sink};
+use std::io::Cursor;
+
+#[test]
+fn test_fresh_cursor_vec() {
+    let mut buf = Cursor::new(b"hello".to_vec());
+
+    assert_eq!(buf.remaining(), 5);
+    assert_eq!(buf.bytes(), b"hello");
+
+    buf.advance(2);
+
+    assert_eq!(buf.remaining(), 3);
+    assert_eq!(buf.bytes(), b"llo");
+
+    buf.advance(3);
+
+    assert_eq!(buf.remaining(), 0);
+    assert_eq!(buf.bytes(), b"");
+
+    buf.advance(1);
+
+    assert_eq!(buf.remaining(), 0);
+    assert_eq!(buf.bytes(), b"");
+}
+
+#[test]
+fn test_get_u8() {
+    let mut buf = Cursor::new(b"\x21zomg");
+    assert_eq!(0x21, buf.get_u8());
+}
+
+#[test]
+fn test_get_u16() {
+    let buf = b"\x21\x54zomg";
+    assert_eq!(0x2154, Cursor::new(buf).get_u16::<byteorder::BigEndian>());
+    assert_eq!(0x5421, Cursor::new(buf).get_u16::<byteorder::LittleEndian>());
+}
+
+#[test]
+#[should_panic]
+fn test_get_u16_buffer_underflow() {
+    let mut buf = Cursor::new(b"\x21");
+    buf.get_u16::<byteorder::BigEndian>();
+}
+
+#[test]
+fn test_vec_sink_capacity() {
+    let mut sink: Vec<u8> = Vec::new();
+    sink.reserve(16);
+    assert!(sink.capacity() >= 16, "Capacity {} must be at least 16", sink.capacity());
+    let mut source = Cursor::new(b"0123456789abcdef0123456789abcdef");
+    sink.sink(&mut source);
+    assert!(sink.len() <= sink.capacity(), "Length {} must be less than or equal to capacity {}", sink.len(), sink.capacity());
+}