Recursively compress output with ** wildcards (fixes #58)

[JuhaKiili]

You can now make calls like:

# relative paths
valohai.outputs('resized').compress("**/*.png", "images.zip", remove_originals=True)

# absolute paths
valohai.outputs('resized').compress("/tmp/foo/**/derp/*.png", "images.zip", remove_originals=True)

And get a meaningful folder structure inside the archive:

train/hello.png
test/jello.png

fixes #58