AlignIO now interleaves (wraps) large Nexus alignments to avoid long lines.

This should allow it to be used with MrBayes, while continuing to
output non-interleaved Nexus for smaller alignments where there
have been no issues reported. See mailing list dicussions:
http://lists.open-bio.org/pipermail/biopython-dev/2010-December/008487.html
http://lists.open-bio.org/pipermail/biopython/2013-March/008457.html