Warn about bad shortcode parameters (and don't crash)

closes #193
Adding an introduction to Hugo
Adding documentation on Table of Contents functionality
Adding documentation on Pretty Urls
A bunch of small documentation changes
Fix imports so hugo builds on darwin
Shortcode cleanup. Added a ton of tests. Much more flexible with input. Doesn't crash with bad input. Fixed #193

Also added the .Get function to short codes and documentation for that function.
Reorganization of helpers. Centralized Url/Path logic. Fixed #175.