Change the name from 'json2js' to 'ng-json2js'

This PR is to avoid confusion while using this preprocessor with others.

There are 4 preprocessors having very similar names: 

- karma-html2js-preprocessor (html2js)
- karma-ng-html2js-preprocessor (ng-html2js)
- karma-json2js-preprocessor (json2js)
- karma-ng-json2js-preprocessor (ng-json2js)

Though `ng-html2js` doesn't have same functionality as `html2js`, `ng-html2js` is exported to karma as 'html2js'.
This makes people comfused.
cf.) https://github.com/karma-runner/karma-ng-html2js-preprocessor/issues/43, https://github.com/karma-runner/karma-ng-html2js-preprocessor/commit/1d3cb4d1c973e35a7653d51833c0e173cf5739bb

`json2js` was created only for solve this confusion. it has almost the same functionality as `html2js`.


Renaming this repo to 'ng-json2js' may help to deal with these problems.

thx!