Fixed For Xcode4.6,customize abbreviation and token height

1.	Fixed: Property ' edgesForExtendedLayout' not found
'edgesForExtendedLayout' is only available in iOS 7.0 or later. If you
compile 'TokenFieldExample' in iOS 6.1 SDK, you should get this error.
You can check for it at compile-time using something like:
#if __IPHONE_OS_VERSION_MAX_ALLOWED >= 70000
    if ([self respondsToSelector:@selector(edgesForExtendedLayout)])
        self.edgesForExtendedLayout = UIRectEdgeNone;
#endif
2.	Add 'abbr' property
I used property 'abbr'  and category ' NSString+truncateToSize' if
users like customize their untoken text like 'Victoria Delgadillo... 4
联系人' rather than default '3 recipients'.

3.	can designate height
By default, the token field's height is fixed to 43 px.
In iPad,it is fine.In iPhone,it is too tall. Maybe we should shorten it
to 24 px so we can save more screen dimensions.
Specify  the frame parameter in ' initWithFrame ' no help for it.I
refined the code so users can customize the token field's height.