Be more flexible with parsing short HEADER records.

This allows parsing of pre-submission PDB files which don't
contain a proper HEADER with PDB ID and deposition date
Fix parsing of SCOP 1.71 (#115)

- Accept some poorly-formatted comment definitions
- Use the first chain found for '_' domains (#115)
- Add tests for 1.71
Fixed bug in latest HEADER parsing change

Does 5928eff8 correctly.