Here's one way to make the hashes match what Maven's plugins would create. It takes a little extra footwork since OS X doesn't have the same command line tools. An alternative is just to make Maven output these of course - would that be better? I ask in case there is a reason I'm missing, like, we need to hash files that Maven doesn't build.

Author: Sean Owen <sowen@cloudera.com>

Closes #4161 from srowen/SPARK-5308 and squashes the following commits:

70d09d0 [Sean Owen] Use $(...) syntax
e25eff8 [Sean Owen] Generate MD5, SHA1 hashes in a format like Maven's plugin