support json marshaller

[mh-cbon]

hi,

currently semver.Version does not support json (un)marshal interfaces.

When json encoded, it results in an empty string.
I suspect it happens because the struct won 't export any public fields.

See the demo code to work around this, alternatively,

package main

import (
	"encoding/json"
	"fmt"

	"github.com/Masterminds/semver"
)

type Jversion struct {
	*semver.Version
}

func NewJversion(s string) (*Jversion, error) {
	v, err := semver.NewVersion(s)
	if err != nil {
		return nil, err
	}
	return &Jversion{Version: v}, nil
}

func (a *Jversion) UnmarshalJSON(b []byte) error {
	var s string
	if err := json.Unmarshal(b, &s); err != nil {
		return err
	}
	v, err := semver.NewVersion(s)
	if err != nil {
		return err
	}
	a.Version = v
	return nil
}

func (a *Jversion) MarshalJSON() ([]byte, error) {
	return json.Marshal(a.String())
}

func main() {
	s, err := NewJversion("0.0.1")
	kk(err)
	b, err2 := json.Marshal(s)
	kk(err2)
	fmt.Println(string(b))
}

func kk(err error) {
	if err != nil {
		panic(err)
	}
}