Chosen more elegant aproach to filling EPublication structure.
Shortened even more; namedtuple's fields are stored in ._fields property.
Safer naming convention in decoders.

With this names they won't rewrite internal module namespace.
DRY.

Created meta_exception.py with MetaParsingException, which is imported
from fields.py and __init__.py.
Added (untested) sketch of meta decoding.