Improved the Shop UI

-Added the Goblin base enemy sprite to the project
-Created a custom GridViewItem style so there are no animations when the user clicks on an item in the Shop
-Changed the template of items in the Shop to display the ShopItem's Name and Description
-Added XAML styles for the ShopItem's Name and Description
-Changed the opacity of the Level End and Shop backgrounds from 0.50 to 0.30 because they were a little too intrusive
-Added public properties for the ShopItem Name and Description in ShopItem.cs
Enemies now have the ability to become invincible upon spawning for a designated period of time

-Moved invincibility variables outside of Player and into LevelObject
Tracked Player Stats in Levels

-Tracked the following stats:

1. Accuracy rate
2. # of enemies the player has killed
3. # of enemies the player's helpers have killed
4. Total kills (#2 + #3)
5. Gold earned in the level
-Displayed the tracked information on the Level End screen
Merge remote-tracking branch 'origin/InvincibleEnemies' into LevelStatsTracking

Conflicts:
	Defend Your Castle/Defend Your Castle/Level.cs
Merge branch 'LevelStatsTracking'