Made the Level of the ShopItem Update on the Shop When it is Purchased

- Made the ShopItem class implement the INotifyPropertyChanged interface
- Changed the LevelString property in ShopItem.cs to a getter and a setter so that it can utilize the INotifyPropertyChanged interface
- Created a method for setting the displayed level of the ShopItem: SetDisplayedLevel
- Used the SetCurrentLevel method for incrementing and decrementing the current level of a ShopItem
- Added a call to the SetDisplayedLevel method to each derived ShopItem's constructor. This was not done in the base class; otherwise, the max level for every ShopItem would be displayed as infinity
- Fixed an issue in the Shop in which the ShopItems under the "Game Saved!" animation would not work after the Save button was pressed once. Made the SaveGameAnimBorder not HitTestVisible to fix this
More balancing work

-Increased the range of archers each level by 75 from 50 and their speed from 200 to 250
-Changed the MaxLevel of UpgradeArcher and UpgradeSlower to 2 instead of 3 since there are only 2 upgrades
-Made 4 enemies spawn starting at Level 34 instead of Level 20
Merge branch 'ShopItemLevelUpdate' into MoreBalancing3
New music playback system

-SoundManager now uses a SoundEffectInstance instead of the MediaPlayer for playing songs

-Increased the damage all enemies, except the ArmoredEnemy, deal by 25
-Made the current song not replay if it was already playing after the player snapped and unsnapped the game
Created Facebook Assets - Profile Picture and Cover Photo
Merge remote-tracking branch 'origin/FacebookAssets' into NewMusicPlayback
Merge branch 'NewMusicPlayback'