Set up dev flow with grunt and fix coverage report

* Added support to debug unminified code with `grunt dev` task. 
* Updated coverage report to be generated based on unminified code and set up thresholds for code coverage. Also, added `grunt coverage` task to allow running it independently from `grunt test`. `grunt test` still generates coverage report and makes sure that coverage thresholds are met (will fail otherwise)
* Updated README.md to reflect new development flow
* Removed custom testRhino task and implemented it as part of shell task instead.
* Updated rhino unit test runner to test dust-full.min.js