Fix handling of uncompressed JAR entries

The Uint8Array returned from zip.read is just an offset into the jar's array
buffer. We need to properly use the byte length and offset when carving out a
slice.
Don't compress classes.jar

By not compressing classes.jar's class files we reduce startup time on mobile
devices by up to 20%.
Merge pull request #486 from EricRahm/uncompressed_zips

Uncompressed zips