Add ImportPreSaleKey

* ImportPreSaleKey takes a KeyStore, a presale key JSON (e.g. file content)
  and a password string. It stores the key in the given key store.
* Refactored common AES decryption and moved some functions to crypto.go

crypto/key_store_test.go

@@ -83,3 +83,16 @@ func TestKeyStorePassphraseDecryptionFail(t *testing.T) {
 		t.Fatal(err)
 	}
 }
+
+func TestImportPreSaleKey(t *testing.T) {
+	// file content of a presale key file generated with:
+	// python pyethsaletool.py genwallet
+	// with password "foo"
+	fileContent := "{\"encseed\": \"26d87f5f2bf9835f9a47eefae571bc09f9107bb13d54ff12a4ec095d01f83897494cf34f7bed2ed34126ecba9db7b62de56c9d7cd136520a0427bfb11b8954ba7ac39b90d4650d3448e31185affcd74226a68f1e94b1108e6e0a4a91cdd83eba\", \"ethaddr\": \"d4584b5f6229b7be90727b0fc8c6b91bb427821f\", \"email\": \"gustav.simonsson@gmail.com\", \"btcaddr\": \"1EVknXyFC68kKNLkh6YnKzW41svSRoaAcx\"}"
+	ks := NewKeyStorePassphrase(DefaultDataDir())
+	pass := "foo"
+	_, err := ImportPreSaleKey(ks, []byte(fileContent), pass)
+	if err != nil {
+		t.Fatal(err)
+	}
+}