[JENKINS-21419] Can't open sloccount table portlet

- The code used System.getProperty("file.separator") for detection of unix/windows directory separator. This technique may be dangerous now, because the SLOCCount report (output from sloccount utility) may be generated on slave (unix/windows), the same file then transfered to master (windows/unix) and lazy loaded/parsed on master. So both unix and windows separators should be considered during the parsing.
- Directory separator member variables removed from SloccountReport, System.getProperty("file.separator") is no longer used, because it lost its sense and may return not suitable value.
- Method extractFolder() tests unix separator first and then the windows one. If there is no match, empty string will be returned. -1/not found returned from lastIndexOf() will never cause StringIndexOutOfBoundsException.
- Method updateRootFolderPath() updated to try unix separator first and then the windows one.
- Unit/integration tests for the changes above created.
- Tested only on Linux, I don't have Windows machine available.
- This commit is partially inspired by André Wegmüller's two years old pull request. The code is now too incompatible, sorry.
- If no file is matching the input pattern, the publisher will output a warning to the build log.
[JENKINS-21419] Can't open sloccount table portlet

- Links to specific folders in folder details don't work.
- All Windows backslashes are now replaced by Unix slashes at the very beginning. Only unix slashes are considered in the rest of the code, which significantly simplifies it and prevents possible errors.
- SloccountReport.extractFolder() is now used in FolderFileFilter.include() to fix the same issue that was present in extractFolder().
- Constant DIRECTORY_SEPARATOR constant is used instead of hardcoded "/".
Merge pull request #26 from mixalturek/master

Fixes for Windows