Move JSONInit class to param?

[jlstevens]

In PR #52, the JSONInit class is added to parambokeh. Instead of duplicating this class across paramnb and parambokeh, it might be worth moving this class and the corresponding documentation somewhere else (probably to param?).

[jbednar]

I think it's definitely worth it, as part of general support for configuration in Param, which is related to the yaml and JSON configuration file reading support currently in holoviz/param#153 . But let's work it out here first, then graduate it to Param separately.