cache_file_name and json_file_name are not consistent or descriptive

[dwikler]

cache_file_name and json_file_name are not consistent and descriptive enough.
This may confuse API users.

cache_file_name is used for caching the raw downloaded XHTML/PDF file.
json_file_name is used for caching the parsed/cached model (JSON).

Caching is expressed in one of them where format is expressed in the the other.
Using format name is not extensible to other formats SpecStore subclasses.

Suggestions for improvement:

1. Use more explicit names:
   input_cache_file for the raw downloaded file (XHTML/PDF).
   model_cache_file parsed/cached model.