Fix docstring: problems problems -> problems
setup.py: Import information from module
Add basic string conversion for games
Add some constants for NAGs
Allow excluding comments, variations and headers
Add concept of PGN exporter

Exporters get to see games with headers and their tree of variations in a
flattened way.

They may encapsulate subtle formatting changes, like maximum columns
widths and they can also have different targets like a string or a file.
Implement file exporter
Test and fix string exporter
Add exporter.put_result()