Change declaration from assign to weak

This will nil-out the delegate when deallocated.
This prevents a crash that happens if the delegate is deallocated before the button-hiding animation completes

This was issue:
Animation Timers after delegate deallocated #33
Merge pull request #34 from walsh2000/master

Change declaration from assign to weak
Merge pull request #29 from demdxx/master

Added layer.contents use for expansion button background
Fixed issue #35 and issue #32: Swipe button can respond even if it is hidden
Fixed issue #31: Swipe doesn't work for cached cells after data reload
Updated pod spec
Fixed issue #37 : Right expansion animation not working properly
Improved expansion animation for button indexes > 0: hide buttons behind expanded buttons
Fixed duplicated expansion backgrounds race conditions
Found a better way to fix issue #37 : Right expansion animation not working properly
Fixed issue #36 : Problem with swipe gesture after I call showSwipe
updated podspec
updated podspec
Fixed issue #38: Application crash on tap
Fixed table scroll while swipping
updated podspec