Allow empty & Retina fix

Hey Jan!

Two main changes here:

1. We've been running into issues trying to stamp small icons, we'd like to add an `--allow-empty` option to just ignore failures if the text doesn't fit.
2. The integration tests were broken on my machine because of #1. So I fixed that too.

Had to switch `BRLOptionParser` to use the latest commit because it wouldn't compile on Yosemite.

Hope all is well :smiley: