Fix lg:expand cannot retrieve the value from scope with certain paths

[cosmicshuai]

closes: #993
By changing the way of read data from scope and reformation of the variables. Now LG can evaluate expressions like ${user.name} correctly.
For example:

# AddItemReadBack
[Activity
    Text = ${HelpPrefix()}, I have added "**${dialog.itemTitle}**" to your **${dialog.listType}** list. You have **${count(user.lists[dialog.listType])}** items in your ${dialog.listType} list.
    ${WelcomeActions()}
]

# HelpPrefix
- Sure
- You bet
- Absolutely

# WelcomeActions
[Activity
    SuggestedActions = Add item | View lists | Remove item | Profile | Cancel | Help
]

{
    "time":"morning",
    "user": {
        "name": "cosmic",
        "lists": {
            "reminder": ["car washing", "laundry", "cooking"]
        }
    },
    "age": 18,
    "dialog": {
        "itemTitle": "Sunday ToDo",
        "listType": "reminder"
    }
}

will return

# AddItemReadBack
- {"lgType":"Activity","text":"Sure, I have added "**Sunday ToDo**" to your **reminder** list. You have **3** items in your reminder list.","suggestedactions":["Add item ","View lists ","Remove item ","Profile ","Cancel ","Help"]}
- {"lgType":"Activity","text":"You bet, I have added "**Sunday ToDo**" to your **reminder** list. You have **3** items in your reminder list.","suggestedactions":["Add item ","View lists ","Remove item ","Profile ","Cancel ","Help"]}
- {"lgType":"Activity","text":"Absolutely, I have added "**Sunday ToDo**" to your **reminder** list. You have **3** items in your reminder list.","suggestedactions":["Add item ","View lists ","Remove item ","Profile ","Cancel ","Help"]}