Add formula for Plowshare 3

In mxcl/homebrew there is Plowshare version 4 (https://github.com/mxcl/homebrew/blob/master/Library/Formula/plowshare.rb), which requires Bash 4 to run (newer than standard Bash in OS X). This formula installs Plowshare 3, which runs well with Bash 3 and it is still actively supported by upstream.