Readability and maintainability improvement

This PR modifies the entire source code in terms of readability and maintainability.

* Fix all mixed tab/space indentations (This PR unifies them into 2 spaces.)
* Use only single quotes when handling strings (At present, this project uses both single and double quotes.)
* Use as many strict equal operator (===) as possible, instead of (==) operator
* Remove all trailing whitespaces (This project has too many trailing whitespaces now.)
* Remove (or comment out) all unused variables and arguments
* Remove all duplicate declaration of variables