Add connection webhook notifications (#392)

Also 

* Added decode of LoginStart message
* Add metrics backend constants
* Updated usage section
* Documented MaxFrameLength

mcproto/decode.go

@@ -0,0 +1,67 @@
+package mcproto
+
+import (
+	"bytes"
+	"github.com/pkg/errors"
+)
+
+const invalidPacketDataBytesMsg = "data should be byte slice from Packet.Data"
+
+// DecodeHandshake takes the Packet.Data bytes and decodes a Handshake message from it
+func DecodeHandshake(data interface{}) (*Handshake, error) {
+
+	dataBytes, ok := data.([]byte)
+	if !ok {
+		return nil, errors.New(invalidPacketDataBytesMsg)
+	}
+
+	handshake := &Handshake{}
+	buffer := bytes.NewBuffer(dataBytes)
+	var err error
+
+	handshake.ProtocolVersion, err = ReadVarInt(buffer)
+	if err != nil {
+		return nil, err
+	}
+
+	handshake.ServerAddress, err = ReadString(buffer)
+	if err != nil {
+		return nil, err
+	}
+
+	handshake.ServerPort, err = ReadUnsignedShort(buffer)
+	if err != nil {
+		return nil, err
+	}
+
+	nextState, err := ReadVarInt(buffer)
+	if err != nil {
+		return nil, err
+	}
+	handshake.NextState = State(nextState)
+	return handshake, nil
+}
+
+// DecodeLoginStart takes the Packet.Data bytes and decodes a LoginStart message from it
+func DecodeLoginStart(data interface{}) (*LoginStart, error) {
+	dataBytes, ok := data.([]byte)
+	if !ok {
+		return nil, errors.New(invalidPacketDataBytesMsg)
+	}
+
+	loginStart := &LoginStart{}
+	buffer := bytes.NewBuffer(dataBytes)
+	var err error
+
+	loginStart.Name, err = ReadString(buffer)
+	if err != nil {
+		return nil, errors.Wrap(err, "failed to read username")
+	}
+
+	loginStart.PlayerUuid, err = ReadUuid(buffer)
+	if err != nil {
+		return nil, errors.Wrap(err, "failed to read player uuid")
+	}
+
+	return loginStart, nil
+}