rlp: add functions for encoding

I'm reasonably confident that the encoding matches the output of
ethutil.Encode for values that it supports. Some of the tests have been
adpated from the Ethereum testing repository.

There are still TODOs in the code.

rlp/encoder_example_test.go

@@ -0,0 +1,38 @@
+package rlp
+
+import (
+	"fmt"
+	"io"
+)
+
+type MyCoolType struct {
+	Name string
+	a, b uint
+}
+
+// EncodeRLP writes x as RLP list [a, b] that omits the Name field.
+func (x *MyCoolType) EncodeRLP(w io.Writer) (err error) {
+	// Note: the receiver can be a nil pointer. This allows you to
+	// control the encoding of nil, but it also means that you have to
+	// check for a nil receiver.
+	if x == nil {
+		err = Encode(w, []uint{0, 0})
+	} else {
+		err = Encode(w, []uint{x.a, x.b})
+	}
+	return err
+}
+
+func ExampleEncoder() {
+	var t *MyCoolType // t is nil pointer to MyCoolType
+	bytes, _ := EncodeToBytes(t)
+	fmt.Printf("%v → %X\n", t, bytes)
+
+	t = &MyCoolType{Name: "foobar", a: 5, b: 6}
+	bytes, _ = EncodeToBytes(t)
+	fmt.Printf("%v → %X\n", t, bytes)
+
+	// Output:
+	// <nil> → C28080
+	// &{foobar 5 6} → C20506
+}