Fixes Issue #4 Prevent User Interaction And Add iOS 6 compatibility

-Added BackgroundView to HUD to prevent user interaction
-Added conditions to check for iOS7 in order to call iOS7 specific methods only in iOS7 devices and greater

Signed-off-by:Wilferrel
Merge pull request #6 from wilferrel/IOS6Compatiability

Fixes Issue #4 Prevent User Interaction And Add iOS 6 compatibility
Some improvement
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ProgressHUD.podspec
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