Show file size of bundles in verbose mode

If verbose mode is enabled, output the file size as part of writing out
the bundles. Fixes #56

![d4348304-3955-11e4-99a9-88679caa46b2](https://cloud.githubusercontent.com/assets/361671/4656761/3c58eac2-54dd-11e4-8b52-97e276c78c20.png)