prototype.js: Remove unused Math.hypot()

Unused Math.hypot() is no longer needed.
By the way, it may be able to use ES6 Math.hypot() in the near future.
http://people.mozilla.org/~jorendorff/es6-draft.html#sec-20.2.2.16
Move String.katakana from utility.js to prototype.js
Add dev scripts #12

* .gitignore: Add "node_modules/"
Revise prototype.js #12

* Change coding style
* Change indent from 1 tab to 2 spaces
* Use wrapper function
* Use Strict Mode
* Use Object.defineProperties() instead of update()
* Add extendBuiltInObject()
* Custom methods should be non-enumerable
* Gruntfile.js: Uncomment prototype.js
Array#split's step parameter should handle falsy value

* prototype_test_patch.js: Uncomment |['a', 'b', 'c'].split(0)| code test
String#pad should always return "string" type value

* prototype_test_patch.js: Uncomment |(10).pad(0)| and |'a'.pad(0)| code test