Add explicit casts for integer conversion.

Add explicit casts from NSUinteger to uInt for the assignments into
z_stream_s.  These types are different on 64-bit builds, and this
causes a warning in Xcode 5.1 with -Wshorten-64-to-32.
Merge pull request #253 from tipbit/CompressingLogFileManager-explicit-cast

Add explicit casts for integer conversion.