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.

internal/web3ext/web3ext.go

@@ -587,9 +587,20 @@ web3._extend({
 			call: 'personal_sendTransaction',
 			params: 2,
 			inputFormatter: [web3._extend.formatters.inputTransactionFormatter, null]
+		}),
+		new web3._extend.Method({
+			name: 'sign',
+			call: 'personal_sign',
+			params: 3,
+			inputFormatter: [null, web3._extend.formatters.inputAddressFormatter, null]
+		}),
+		new web3._extend.Method({
+			name: 'ecRecover',
+			call: 'personal_ecRecover',
+			params: 2
 		})
 	]
-});
+})
 `
 
 const RPC_JS = `