Simplified the default recipe and url based attributes.

Planning to start adding support for craftbukkit, the start of that
was to simplify how we download the jar file. I took the advise from
@jtimberman's blog post
[here](http://jtimberman.housepub.org/blog/2014/02/01/evolution-of-cookbook-development/),
and went with a single attribute for the download url.