[Radiant] Error and fix for Styles N Scripts extension and/or in Radiant

[jsmorris]

I had to add the following line (@ line 12) to the
styles_n_scripts_extension.rb file to prevent radiant 0.6.7 from
throwing when visiting the extensions page.

...
11:  description "Adds CSS and JS file management to Radiant"
12:  url ""
13:
14:  define_routes do |map|
...

Radiant could not show the list of installed extensions unless styles
n script listed a url.  I think radiant should handle this better, but
styles n script extension should add an appropriate value.

Jason