Handle URLs with parameters more sensibly.

Shellquote is the closest thing to array-of-arguments exec calls puppet
supports. As far as I've ever been able to test, it correctly handles quoting
for the shells Puppet uses for `exec{}` resources. Using it to construct
`curl`/`wget` commands means that they correctly download, for example, Jetty:

    http://eclipse.org/downloads/download.php?file=/jetty/stable-8/dist/jetty-distribution-8.1.14.v20131031.tar.gz&r=1

Without shellquote, the `exec` resources see the embedded `&` near the end
and generate two shell commands (the second of which is `r=1`). With
shellquote, the URL is kept as a single argument.
Merge pull request #15 from ojacobson/shellquote

Handle URLs with parameters more sensibly.