Tar: Correct and test modification time

- Read modification time for folders as well;
- Avoid a range error when serializing timestamp;
- Remove memory allocation when serializing timestamp and simplify code by using sformat;
- Add unittest for modification time;

GzFilter now supports different compression levels.

GzFilter now supports different compression levels.

GzFilter now supports different compression levels.

Added 2 examples and uploaded ddoc documentation (fix #11)

Moved files to the archive directory to be compatible with dub builds.

Removed erroneous core file.