Removes strong references to self in blocks

We were noticing issues where even when a memory warning was triggered the memory foot print never went down or went down a trivial amount.

By removing strong references to self this problem appears to be gone. However there are still some memory showing up in the decompress image method when running instruments.

Stack overflow suggests this might be memory used by core graphics and freed separately.

Would appreciate a tag for Cocoapods, I can submit the updated pod to the spec repo if you'd like