Handful of fixes

Background: I'd like to add `json-snatcher` to [MELPA](http://melpa.milkbox.net/). :-)

* Only provide feature after all code has been successfully evaluated
* Declare lexical-binding on the opening line (required for proper effect)
* Rework some docstrings according to checkdoc standards
* Add autoload cookie for interactive function

Some whitespace changes crept into this too, because the initial indentation was inconsistent, but the code itself has not been altered.

One concern, though: there are many references to a function `jsons-put-string`, but a definition is not provided for that function -- might want to fix that. ;-)

Cheers,

-Steve