#13: Added _parse_price() function.
#5: Added binding optional to Publication structure.
#13: All required parameters are now parsed to Publication structure.
#5: Authors are now converted to namedtuple in to_namedtuple().

Renamed property `.author` to `.authors`.
#13: Changes in structures.py.

Added declarations of functions:
- _parse_ISBN()
- _parse_edition()
- _parse_description()