Added progress events, w/ test

We needed to give the user progress notifications when extracting large zips, so they don't give up or think it is stuck. It's a small change, but may prove useful for others.

I'm not super-familiar with the codebase or promises, but I think this should do the trick. I added a test & it passes.

Note that if #37 reduces the number of files in the first test zip file (as requested here in https://github.com/bower/decompress-zip/pull/32#issuecomment-55375832 ?), then numTotalFiles variable in the test will need to be adjusted.