Blast-XML unicode writer fix

See the initial report [here](http://lists.open-bio.org/pipermail/biopython-dev/2013-April/010503.html).

This is the fix to address the unicode writing bug. Travis doesn't have Python2.7.4 up and running yet (and its Py3.x tests unfortunately stalled, last time I checked). So I tested this patch locally (under 2.7.4 and 3.3.1) and the write tests all pass.