Implement FromBuf and Buf::collect

Enables collecting the contents of a `Buf` value into a relevant
concrete buffer implementation.

tests/test_from_buf.rs

@@ -0,0 +1,34 @@
+extern crate bytes;
+
+use bytes::{Buf, Bytes, BytesMut};
+use std::io::Cursor;
+
+const LONG: &'static [u8] = b"mary had a little lamb, little lamb, little lamb";
+const SHORT: &'static [u8] = b"hello world";
+
+#[test]
+fn collect_to_vec() {
+    let buf: Vec<u8> = Cursor::new(SHORT).collect();
+    assert_eq!(buf, SHORT);
+
+    let buf: Vec<u8> = Cursor::new(LONG).collect();
+    assert_eq!(buf, LONG);
+}
+
+#[test]
+fn collect_to_bytes() {
+    let buf: Bytes = Cursor::new(SHORT).collect();
+    assert_eq!(buf, SHORT);
+
+    let buf: Bytes = Cursor::new(LONG).collect();
+    assert_eq!(buf, LONG);
+}
+
+#[test]
+fn collect_to_bytes_mut() {
+    let buf: BytesMut = Cursor::new(SHORT).collect();
+    assert_eq!(buf, SHORT);
+
+    let buf: BytesMut = Cursor::new(LONG).collect();
+    assert_eq!(buf, LONG);
+}