Add javadocs and minor changes

Hello. I'm sorry for the unnecessary commits about gitignore. Some settings were messed up when I updated SourceTree yesterday.

I'm studying PF4J and using the open source framework to create plug-ins at work. I wish I could contribute to the project so started adding JavaDocs while studying the source code. 

I apologize for messing with indentation in some files. I was using Eclipse with 120-ish characters per line and 2-spaces instead of tab character. I will change the Eclipse formatter setting if you want me to follow your coding style.

There is no major changes in these commits, only minor JavaDoc addition, getting rid of repetitive calls, etc.

Changes are ...
- Adding 'bin' folder in gitignore (Eclipse project).
- Replacing hardcoded constant 'UTF-8' in DefaultExtensionFinder.
- Replacing multiple calls of getName() in Find method in DefaultExtensionFinder.
- Adding @SuppressWarnings("unchecked") in ExtensionWrapper