Use relative symlinks for single color tiles

Using a relative link allows the cache to be portable even when the
option to link single color tiles is used.
Use relative symlink when os.path.relpath is available

Python 2.5 does not support os.path.relpath so use an absolute path for
2.5 and relative >= 2.6.
Merge pull request #107 from walkermatt/symlink

Use relative symlinks for single color tiles