Automated UDT support

This patch adds the power of GocqlX to UDTs.
Now you can make a struct be UDT compatible by adding a single line.

```
type FullName struct {
	gocqlx.UDT
	FirstName string
	LastName  string
}
```

Signed-off-by: Michał Matczuk <michal@scylladb.com>

udt.go

@@ -0,0 +1,74 @@
+// Copyright (C) 2017 ScyllaDB
+// Use of this source code is governed by a ALv2-style
+// license that can be found in the LICENSE file.
+
+package gocqlx
+
+import (
+	"fmt"
+	"reflect"
+
+	"github.com/gocql/gocql"
+	"github.com/scylladb/go-reflectx"
+)
+
+// UDT is a marker interface that needs to be embedded in a struct if you want
+// to marshal or unmarshal it as a User Defined Type.
+type UDT interface {
+	udt()
+}
+
+var (
+	_ gocql.UDTMarshaler   = udt{}
+	_ gocql.UDTUnmarshaler = udt{}
+)
+
+type udt struct {
+	value  reflect.Value
+	field  map[string]reflect.Value
+	unsafe bool
+}
+
+func makeUDT(value reflect.Value, mapper *reflectx.Mapper, unsafe bool) udt {
+	return udt{
+		value:  value,
+		field:  mapper.FieldMap(value),
+		unsafe: unsafe,
+	}
+}
+
+func (u udt) MarshalUDT(name string, info gocql.TypeInfo) ([]byte, error) {
+	value, ok := u.field[name]
+	if !ok {
+		return nil, fmt.Errorf("missing name %q in %s", name, u.value.Type())
+	}
+
+	return gocql.Marshal(info, value.Interface())
+}
+
+func (u udt) UnmarshalUDT(name string, info gocql.TypeInfo, data []byte) error {
+	value, ok := u.field[name]
+	if !ok && !u.unsafe {
+		return fmt.Errorf("missing name %q in %s", name, u.value.Type())
+	}
+
+	return gocql.Unmarshal(info, data, value.Addr().Interface())
+}
+
+// udtWrapValue adds UDT wrapper if needed.
+func udtWrapValue(value reflect.Value, mapper *reflectx.Mapper, unsafe bool) interface{} {
+	if value.Type().Implements(autoUDTInterface) {
+		return makeUDT(value, mapper, unsafe)
+	}
+	return value.Interface()
+}
+
+// udtWrapSlice adds UDT wrapper if needed.
+func udtWrapSlice(mapper *reflectx.Mapper, unsafe bool, v []interface{}) []interface{} {
+	for i := range v {
+		if _, ok := v[i].(UDT); ok {
+			v[i] = makeUDT(reflect.ValueOf(v[i]), mapper, unsafe)
+		}
+	}
+	return v
+}