SeqIO: Better cope with wrong LOCUS line spacing in GenBank

Rebased on complementary patches from @peterjc
Includes the warning requested in https://github.com/biopython/biopython/pull/187#issuecomment-19538248

Supersedes the old pull request 187.

Signed-off-by: Kai Blin <kai.blin@biotech.uni-tuebingen.de>