internal/ethapi: add personal_sign and fix eth_sign to hash message (#2940)

This commit includes several API changes:

- The behavior of eth_sign is changed. It now accepts an arbitrary
  message, prepends the well-known string

        \x19Ethereum Signed Message:\n<length of message>

  hashes the result using keccak256 and calculates the signature of
  the hash. This breaks backwards compatability!
  
- personal_sign(hash, address [, password]) is added. It has the same
  semantics as eth_sign but also accepts a password. The private key
  used to sign the hash is temporarily unlocked in the scope of the
  request.
  
- personal_recover(message, signature) is added and returns the
  address for the account that created a signature.

console/console.go

@@ -156,10 +156,9 @@ func (c *Console) init(preload []string) error {
 		if err != nil {
 			return err
 		}
-		// Override the unlockAccount and newAccount methods since these require user interaction.
-		// Assign the jeth.unlockAccount and jeth.newAccount in the Console the original web3 callbacks.
-		// These will be called by the jeth.* methods after they got the password from the user and send
-		// the original web3 request to the backend.
+		// Override the unlockAccount, newAccount and sign methods since these require user interaction.
+		// Assign these method in the Console the original web3 callbacks. These will be called by the jeth.*
+		// methods after they got the password from the user and send the original web3 request to the backend.
 		if obj := personal.Object(); obj != nil { // make sure the personal api is enabled over the interface
 			if _, err = c.jsre.Run(`jeth.unlockAccount = personal.unlockAccount;`); err != nil {
 				return fmt.Errorf("personal.unlockAccount: %v", err)
@@ -167,8 +166,12 @@ func (c *Console) init(preload []string) error {
 			if _, err = c.jsre.Run(`jeth.newAccount = personal.newAccount;`); err != nil {
 				return fmt.Errorf("personal.newAccount: %v", err)
 			}
+			if _, err = c.jsre.Run(`jeth.sign = personal.sign;`); err != nil {
+				return fmt.Errorf("personal.sign: %v", err)
+			}
 			obj.Set("unlockAccount", bridge.UnlockAccount)
 			obj.Set("newAccount", bridge.NewAccount)
+			obj.Set("sign", bridge.Sign)
 		}
 	}
 	// The admin.sleep and admin.sleepBlocks are offered by the console and not by the RPC layer.