Declare ACC flags constants

- Add convenience methods for a few accessFlag checks

- Rename hasSuperClass() to isSubclass()
Refactor listFiles

- Remove recursive calls

- Check isDirectory() inside listFiles() and return empty
  list when false
Fix javadoc

Use 3rd person (descriptive) not 2nd person (prescriptive)
Log stack trace

- Ignore exception in closeQuietly
Rename check() => usesClass()

Refactor duplicated code and replace with addUsages() method
Replace indexOf with last char comparison
Extract constant pool tag constants