Add support for pulling down dependencies via curl

OS X doesn't have wget installed by default. Despite the fact that most
of us have figured out a way to install it, I'd like to add support for
curl so that we can make one less inconvenience for those who don't have
wget installed.
Add download() to build-solr.sh.
Merge pull request #361 from basho/hc/curl-and-wget

Add support for pulling down dependencies via curl

Reviewed-by: coderoshi