Merge branch 'master', remote-tracking branch 'origin'
Downgrade non-null SFF padding error to a warning (issue #219).

Based on a patch from Martin Mokrejs, this allows the
Biopython SFF parser to be used on invalid SFF files
generated by sff-dump.2.1.7 though 2.1.9 when converting
from *.sra files (problem was due to reuse of an output
buffer, fixed in release 2.1.10).

Martin tested this on SRR088820.sff, ERR016587.sff, and
ERR016602.sff from the SRA.