refactor: Large refactor of the middleware, mostly extracting functions and clarifying names.

Extract helper functions:
* is_cachified_uri
* remove_prefix
* get_true_path
* check_exists
* get_expected_hash
* do_contents_match_expected_hash
* clean_options
* set_locals
* set_cache_headers
* remove_other_cache_headers
refactor: More cleanup and simplification.

* Rename `opts` to `_opts` to be consistent with other global variables.

Add more helper functions:
* does_file_match_expected_hash
* get_uri
* should_skip_hashification
* get_predefined_hash
* url_to_filename
* get_resource_hash
* generate_hashed_url
* should_use_development_resources
* generate_tags_for_resources
* rename prod_or_dev_tags to generate_tags_for_filename
Drop node 0.6 from testing, add node 0.10
Merge pull request #43 from mozilla/refactor-simplify-extract

Refactor simplify extract