loadJSON fix

loadJSON fix:
- this fixes the issue #425 where some json api calls were not loading.

-  jsonp works if the loadJSON path ends with 'callback=nameOfCallback'. It still throws an error from the reqwest library, though, related to this issue https://github.com/ded/reqwest/issues/132) where lastValue is undefined. My [fix](https://github.com/therewasaguy/p5.js/compare/loadjsonfix?expand=1#diff-d1e3eb8e0ad33e6757c4575552774515R57) is kind of a [hack](http://stackoverflow.com/questions/359788/how-to-execute-a-javascript-function-when-i-have-its-name-as-a-string).

- adding a jsonp example