Improved handling of symbolic links for both files and directories

Hello again,

Here's my pull request for adding full support of symbolic links to mr.bob.  Symbolic links are replicated exactly as per source with their names rendered and .bob removed from any files.

I also cleaned up the render_structure function a little while I was working on this with more consistent variable names and PEP8 compliance :smile: 

More to come soon!
Fotis