Bug Fixed StartPointValidator

No of start points must be exactly One.
Bug Fix EndPoint Validator
Import Warning Fixed for Validators
PathLengthValidator() 

This method checks the length of the path.
It handles the case when the user tries to select start and end point
connected with each other.
PlayingScene Error fixed

PlayingScene Error fixed
implement the road title change
Merge pull request #42 from joeyzhang1989/master

implement the road title change
implement the button for tower
Merge pull request #43 from joeyzhang1989/master

implement the button for tower
implement the scenery buttons
Merge pull request #44 from joeyzhang1989/master

implement the scenery buttons
initial implement build tower
Merge pull request #45 from joeyzhang1989/master

initial implement build tower
change the MainScene class
Merge pull request #47 from alexwolfwang/master

change the MainScene class
delete the useless info in MainScene
change the comments in MainScene
add getter setter
Merge pull request #48 from alexwolfwang/master

delete the useless info in MainScene
Refactored MapItem

Refactored MapItem