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.

common/bytes.go

@@ -37,7 +37,7 @@ func ToHex(b []byte) string {
 
 func FromHex(s string) []byte {
 	if len(s) > 1 {
-		if s[0:2] == "0x" {
+		if s[0:2] == "0x" || s[0:2] == "0X" {
 			s = s[2:]
 		}
 		if len(s)%2 == 1 {