make sure playlist objects always contain the entries field

fixes #46

store/types.go

@@ -221,7 +221,7 @@ type Playlist struct {
 	PlayoutMode string          `json:"playout-mode" gorm:"not null;default:'linear'"`
 	ShowName    string          `json:"show" gorm:"not null;index"`
 	Show        Show            `json:"-" gorm:"association_foreignkey:Name"`
-	Entries     []PlaylistEntry `json:"entries,omitempty"`
+	Entries     []PlaylistEntry `json:"entries"`
 }
 
 //****************