Add RPC Server method ValidateAddress (#134)

* Add RPC Server method ValidateAddress

- implement rpc method validateaddress (https://docs.neo.org/en-us/node/cli/2.9.4/api/validateaddress.html)
- add tests
- add to README.md

* revert go.sum

* remove break

* more tests and C# errors

* simplify

* fix after master merge

pkg/rpc/wrappers/validate_address.go

@@ -0,0 +1,21 @@
+package wrappers
+
+import (
+	"github.com/CityOfZion/neo-go/pkg/crypto"
+)
+
+type ValidateAddressResponse struct {
+	Address interface{} `json:"address"`
+	IsValid bool        `json:"isvalid"`
+}
+
+// ValidateAddress verifies that the address is a correct NEO address
+// see https://docs.neo.org/en-us/node/cli/2.9.4/api/validateaddress.html
+func ValidateAddress(address interface{}) ValidateAddressResponse {
+	resp := ValidateAddressResponse{Address: address}
+	if address, ok := address.(string); ok {
+		_, err := crypto.Uint160DecodeAddress(address)
+		resp.IsValid = err == nil
+	}
+	return resp
+}