Implement soft references object for use in caching package

[mpwoodward]

(Moved from http://trac.mach-ii.com/machii/ticket/70)

We should consider implementing a generic soft reference object in MachII.util that could be used by the time span cache strategy and other other potential system that requires a soft reference registration.

This would be important to alleviate any problems with Java throwing an OutOfMemoryError if there is too much too loaded up in a cache.