Move StringUtils and UserAgent classes to utils

They are tools and all provide static methods.
Add FileUtils

Provide some methods to operate on files.
Fix a bug in HelperLauncher

Some times coreThread is null, call interrupt() will cause NullPointer
exceptions, check it before use it.
Fix CoreInterface class

cleanFiles sounds better than clearFiles
Modified DefaultCore classes

Remove clearFiles method;
Add more methods to the main thread to provide more functions.