Sign-error fix by michael.z, from StackOverflow

This fixes a crashing bug in file decompression, caused by improper
negation of an unsigned variable.
http://stackoverflow.com/questions/16802489/zdelta-compression-library-segfaults
Added a new C-block-based convenience API

This is handy for people using Clang, or other compilers (are there
any?) that support blocks. It lets you create or apply deltas and
consume the output incrementally, i.e. writing it to a file or socket,
instead of buffering the entire output in memory.
(This is my own contribution, not something I got from somewhere else.)
Bumped ZDLIB_VERSION to 2.1.1
Added Cocoa/Objective-C API

Some category methods on NSData for creating/applying deltas.