Adapt Json\Json for better prettyPrint support

I adapted the Json::prettyPrint to generate the same output as json_encode

also I added a "prettyPrint" option to Json::encode to take advantage of the json_encode pretty print and align the functionality to json_encode

This is my first pull request ever, so please be kind :-)
Why not check if JSON_PRETTY_PRINT is defined?
Now one must know the exact vesion of PHP that introduced this constant.
And what about hhvm - they may handle it a bit different (and may not, they for example have some problems with T_FINALLY).


Check if the feature you want is in the environment, not if the environment version is as you want.