Use strict mode, add jshintrc and enforce jshint
Define onerror callback if not specified
Merge pull request #67 from Rob--W/master

Define missing onerror callback
Use a single web worker for all tasks in a ZipReader/ZipWriter.
Separate codecs (Inflater/Deflater) and worker codes, and allow pluggable codecs.
Add codecs wrappers for zlib-asm and pako, as replacements for default codecs(deflate.js/inflate.js).
Update README for worker improvements and zlib-asm/pako.
Fix data duplication bug of pako/codecs.js
Move crc32 into worker, further reduce main thread's load.
Update README.md
Update README.md
Coding style improvements