adding support for lang query param for prefix switching

@drsnyder 
Adding support for lang query param, which will change the prefix when searching for an image. Required because different-language-but-same-wiki wikis (like answers) share the same bucket. Ex:

s3://answers/es/images/0/00/Bella_calamidades.jpg
s3://answers/images/0/00/Bahrain_flag.svg
This is starting to get messy. Previously we were working with one level of abstraction on either the `thumb-map` or `this` but now we are adding an additional level of abstraction with the call to `q/query-opts->image-prefix`.

I think this should be refactored to keep these blocks simple and reduce the duplication. For example, maybe the `put*` signature should be `(put* store resource thumb-map)` and let it handle the path generation.

Thoughts?