Groovy download url to file

This has the same interface as the original JsonSlurper written for version 1.8.0, but its implementation has completely changed. It is now up to 20x faster than before, and its speed competes and often substantially exceeds popular common JSON parsers circa Jan, 2014.

The first is to use relative path uri, the second is to use file: url, and the last is to classpath: uri -->