Fixed memory management issues and backwards compatibility.

In ASBSparkLineView.m:
	- Several properties were listed as retain in the header file, but the setter methods did not properly release the old value and retain the new one. The properties that were listed as copy also did not release the old value (but do not need to retain the new one). Changes were made to setter methods for the following properties:
		- dataValues
		- labelText
		- labelColor
		- currentValueColor
		- currentValueFormat
		- rangeOverlayColor
		- rangeOverlayLowerLimit
		- rangeOverlayUpperLimit
	- I updated the init method to ensure that all properties are initially alloc'd and init'd (so that we never try to release an uninitiated pointer), and the dealloc method to ensure that they are all released upon object destruction.

In ASBSparkLineView.h:
	- I changed the terminology from 'strong' to 'retain' and 'weak' to 'assign'. Older compilers don't support 'strong' and 'weak', and my understanding is that they are effectively equivalent (for these purposes at least) on newer compilers.