Bug fixes, content inset and kCAMediaTimingFunctionDefault

• crash that could happen if animation was running on dealloc'ed button
• merge pull request from ehlersd, content inset support in the button.
• Changed all timing functions to kCAMediaTimingFunctionDefault instead of the linear or easeInOut I was using. In iOS7, the button transition and highlight/unhighlight animations are now using the more natural spring animation used throughout the OS.