Added general file access and decoder API.

- This API allows anyone to register a decoder to decode any type of
file into one or more subfiles. See rave/decoder.py for documentation.
- rave/loader.py provides a general API to get a full file listing from
all files and decoded files, and read from and write to them.
- Made LICENSE.md a bit more Markdown-compliant.
- Fixed some small documentation errors.