Improve creating of releases

Get `hub release create --attach A_FILE` and `hub release create --attach A_FOLDER` to work. I reworked @calavera's previous commit https://github.com/github/hub/commit/c20cdc40001c74bb3d0b4a805c66ecdd3a2449c6 to make the code more readable.

It prob. make sense to add some cucumber specs as well.
"from a directory" bit is now outdated docs.