Read in static files into `collection.files` as `StaticFile`s.

Fixes #2555.
> In Ruby, symbols are never garbage collected after they are created. A symbol always points to the same memory address — that’s what makes them so good for repeated hash keys. They don’t use new memory or have to be garbage collected!.
> 
> But that’s a double-edged sword. If too many symbols are created they may exahust physical memory.

http://www.sitepoint.com/rails-security-pitfalls/