Fixed NoSuchMethodError on all devices prior to JellyBean (SDK 16)

In LegacyDialog the call to setBackground() caused a NoSuchMethodError since prior to JellyBean this method was names setBackgroundDrawable().
Changed the method name to fit the overall style
Merge pull request #17 from writtmeyer/fix_nosuchmethoderror

Fix for NoSuchMethodError on devices < API level 16