better patching for zip files

Adds a zip mode ("-z") to imgdiff to construct efficient patches for
zip files (including jars and apks).  We identify the regions within
the zip file containing deflated data, and when a corresponding file
can be found in the source zip, a patch is generated for the
uncompressed version of the data.

The GZIP chunk type is replaced with a DEFLATE chunk type that handles
a raw deflated data stream.  This new DEFLATE chunk can be used for
both gzipped pieces (as found within boot and recovery images) and zip
files (apks, etc.)  The gzip header and footer are handled by NORMAL
chunks on either side of the main DEFLATE chunks.  (Typically these
tiny NORMAL chunks will get merged with adjacent chunks, so the number
of output chunks is unaffected.)

Add a test script that tests the generate-apply cycle on all the zips
and images within a pair of full OTA packages.