Add Option for Relative Paths in Symbolic Links

By default the symbolic links are absolute paths. This change adds an
option to use relative paths. The path is calculated with os.path.relpath.
Merge branch 'master' of github.com:kaelspencer/mmirror

Conflicts:
	mmirror.py
Update README With --relative Information

Extend the readme to include information about relative targets for
the symbolic links.