Make Usages agnostic to file extension

Allow any Java-based file formats to be read no matter what
their extension is. The only requirement should be that the
source files are compiled into .class files.
Clean up logging

- Print affected files using the lifecycle logging level to make it
  consistent with the "no affected files" case

- Consistently use two space indentation for "sub-logging" info
  log messages
Fix bug where 'this' class is not checked for usage

When checking the usage of a file, the class itself should be
checked in addition to classes it uses.
Revert signature of classNameToPath
Merge pull request #29 from jle/support_groovy_file_ext

Allow any file extension